Document: draft-cheshire-dnsext-multicastdns-08.txt Stuart Cheshire
Internet-Draft Marc Krochmal
Category: Informational Apple Inc.
Expires: 10 March 2010 10 September 2009
Multicast DNS
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 10th March 2010.
Abstract
As networked devices become smaller, more portable, and
more ubiquitous, the ability to operate with less configured
infrastructure is increasingly important. In particular,
the ability to look up DNS resource record data types
(including, but not limited to, host names) in the absence
of a conventional managed DNS server, is becoming essential.
Multicast DNS (mDNS) provides the ability to do DNS-like operations
on the local link in the absence of any conventional unicast DNS
server. In addition, mDNS designates a portion of the DNS namespace
to be free for local use, without the need to pay any annual fee, and
without the need to set up delegations or otherwise configure a
conventional DNS server to answer for those names.
The primary benefits of mDNS names are that (i) they require little
or no administration or configuration to set them up, (ii) they work
when no infrastructure is present, and (iii) they work during
infrastructure failures.
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Table of Contents
1. Introduction....................................................3
2. Conventions and Terminology Used in this Document...............3
3. Multicast DNS Names.............................................5
4. Source Address Check............................................9
5. Reverse Address Mapping........................................10
6. Querying.......................................................11
7. Duplicate Suppression..........................................16
8. Responding.....................................................18
9. Probing and Announcing on Startup..............................24
10. Conflict Resolution............................................30
11. Resource Record TTL Values and Cache Coherency.................32
12. Special Characteristics of Multicast DNS Domains...............38
13. Multicast DNS for Service Discovery............................39
14. Enabling and Disabling Multicast DNS...........................39
15. Considerations for Multiple Interfaces.........................40
16. Considerations for Multiple Responders on the Same Machine.....41
17. Multicast DNS and Power Management.............................43
18. Multicast DNS Character Set....................................45
19. Multicast DNS Message Size.....................................47
20. Multicast DNS Message Format...................................48
21. Choice of UDP Port Number......................................52
22. Summary of Differences Between Multicast DNS and Unicast DNS...53
23. Benefits of Multicast Responses................................54
24. IPv6 Considerations............................................55
25. Security Considerations........................................56
26. IANA Considerations............................................57
27. Acknowledgments................................................57
28. Deployment History.............................................57
29. Copyright Notice...............................................58
30. Normative References...........................................59
31. Informative References.........................................59
32. Authors' Addresses.............................................61
Summary of Changes Since draft-cheshire-dnsext-multicastdns-07.txt
The notable changes in this draft compared to draft-7 are:
o Based on feedback from the DNSEXT working group, we updated the
Negative Responses section to use existing DNS record type 'NSEC'
instead of inventing a new pseudo-RR type 'NEGATIVE'.
o Updated Power Management (Sleep Proxy) section
We do not anticipate any further substantive changes to the protocol.
Indeed, even these changes do not break compatibility with previous
implementations. The protocol as described in this document remains
fully compatible with Multicast DNS as shipped by Apple in Mac OS X
10.2 in 2002, and remains fully compatible with network printers and
other devices from that era that implement Multicast DNS.
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1. Introduction
When reading this document, familiarity with the concepts of
Zero Configuration Networking and automatic link-local addressing
[RFC 2462] [RFC 3927] is helpful.
This document proposes no change to the structure of DNS messages,
and no new operation codes or response codes, or resource record
types. This document discusses what needs to happen if DNS clients
start sending DNS queries to a multicast address, and how a
collection of hosts can cooperate to collectively answer those
queries in a useful manner.
There has been discussion of how much burden Multicast DNS might
impose on a network. It should be remembered that whenever IPv4 hosts
communicate, they broadcast ARP packets on the network on a regular
basis, and this is not disastrous. The approximate amount of
multicast traffic generated by hosts making conventional use of
Multicast DNS is anticipated to be roughly the same order of
magnitude as the amount of broadcast ARP traffic those hosts already
generate.
Applications making new use of Multicast DNS capabilities for new
purposes will inevitably generate more traffic. For example, also
using Multicast DNS for Service Discovery [DNS-SD] would be expected
to generate more traffic than using Multicast DNS for hostname
resolution alone. It is reasonable to consider this additional
Service Discovery traffic separately from hostname resolution
traffic, since some other multicast-based Service Discovery protocol
would in any case be generating multicast traffic of its own.
It is possible that some new applications layered on top of Multicast
DNS might be "chatty", and in that case work will be needed to help
them become less chatty. When performing any analysis, it is
important to make a distinction between the application behavior and
the underlying protocol behavior. If a chatty application uses UDP,
that doesn't mean that UDP is chatty, or that IP is chatty, or that
Ethernet is chatty. What it means is that the application is chatty.
The same applies to any future applications that may decide to layer
increasing portions of their functionality over Multicast DNS.
2. Conventions and Terminology Used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in "Key words for use in
RFCs to Indicate Requirement Levels" [RFC 2119].
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This document uses the term "host name" in the strict sense to mean a
fully qualified domain name that has an IPv4 or IPv6 address record.
It does not use the term "host name" in the commonly used but
incorrect sense to mean just the first DNS label of a host's fully
qualified domain name.
A DNS (or mDNS) packet contains an IP TTL in the IP header, which
is effectively a hop-count limit for the packet, to guard against
routing loops. Each Resource Record also contains a TTL, which is
the number of seconds for which the Resource Record may be cached.
In any place where there may be potential confusion between these two
types of TTL, the term "IP TTL" is used to refer to the IP header TTL
(hop limit), and the term "RR TTL" is used to refer to the Resource
Record TTL (cache lifetime).
When this document uses the term "Multicast DNS", it should be taken
to mean: "Clients performing DNS-like queries for DNS-like resource
records by sending DNS-like UDP query and response packets over IP
Multicast to UDP port 5353."
This document uses the terms "shared" and "unique" when referring to
resource record sets:
A "shared" resource record set is one where several Multicast DNS
Responders may have records with that name, rrtype, and rrclass, and
several Responders may respond to a particular query.
A "unique" resource record set is one where all the records with
that name, rrtype, and rrclass are conceptually under the control
or ownership of a single Responder, and it is expected that at most
one Responder should respond to a query for that name, rrtype, and
rrclass. Before claiming ownership of a unique resource record set,
a Responder MUST probe to verify that no other Responder already
claims ownership of that set, as described in Section 9.1 "Probing".
For fault-tolerance and other reasons it is permitted sometimes to
have more than one Responder answering for a particular "unique"
resource record set, but such cooperating Responders MUST give
answers containing identical rdata for these records or the
answers will be perceived to be in conflict with each other.
Strictly speaking the terms "shared" and "unique" apply to resource
record sets, not to individual resource records, but it is sometimes
convenient to talk of "shared resource records" and "unique resource
records". When used this way, the terms should be understood to mean
a record that is a member of a "shared" or "unique" resource record
set, respectively.
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3. Multicast DNS Names
This document specifies that the DNS top-level domain ".local."
is a special domain with special semantics, namely that any fully-
qualified name ending in ".local." is link-local, and names within
this domain are meaningful only on the link where they originate.
This is analogous to IPv4 addresses in the 169.254/16 prefix, which
are link-local and meaningful only on the link where they originate.
Any DNS query for a name ending with ".local." MUST be sent
to the mDNS multicast address (224.0.0.251 or its IPv6 equivalent
FF02::FB).
It is unimportant whether a name ending with ".local." occurred
because the user explicitly typed in a fully qualified domain name
ending in ".local.", or because the user entered an unqualified
domain name and the host software appended the suffix ".local."
because that suffix appears in the user's search list. The ".local."
suffix could appear in the search list because the user manually
configured it, or because it was received in a DHCP option [RFC
2132], or via any other valid mechanism for configuring the DNS
search list. In this respect the ".local." suffix is treated no
differently to any other search domain that might appear in the DNS
search list.
DNS queries for names that do not end with ".local." MAY be sent to
the mDNS multicast address, if no other conventional DNS server is
available. This can allow hosts on the same link to continue
communicating using each other's globally unique DNS names during
network outages which disrupt communication with the greater
Internet. When resolving global names via local multicast, it is even
more important to use DNSSEC or other security mechanisms to ensure
that the response is trustworthy. Resolving global names via local
multicast is a contentious issue, and this document does not discuss
it in detail, instead concentrating on the issue of resolving local
names using DNS packets sent to a multicast address.
A host that belongs to an organization or individual who has control
over some portion of the DNS namespace can be assigned a globally
unique name within that portion of the DNS namespace, for example,
"cheshire.apple.com." For those of us who have this luxury, this
works very well. However, the majority of home computer users do not
have easy access to any portion of the global DNS namespace within
which they have the authority to create names as they wish. This
leaves the majority of home computers effectively anonymous for
practical purposes.
To remedy this problem, this document allows any computer user to
elect to give their computers link-local Multicast DNS host names of
the form: "single-dns-label.local." For example, a laptop computer
may answer to the name "cheshire.local." Any computer user is granted
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the authority to name their computer this way, provided that the
chosen host name is not already in use on that link. Having named
their computer this way, the user has the authority to continue using
that name until such time as a name conflict occurs on the link which
is not resolved in the user's favor. If this happens, the computer
(or its human user) SHOULD cease using the name, and may choose to
attempt to allocate a new unique name for use on that link. These
conflicts are expected to be relatively rare for people who choose
reasonably imaginative names, but it is still important to have a
mechanism in place to handle them when they happen.
The point made above is very important and bears repeating.
It is easy for those of us in the IETF community who run our own
name servers at home to forget that the majority of computer users
do not run their own name server and have no easy way to create their
own host names. When these users wish to transfer files between two
laptop computers, they are frequently reduced to typing in
dotted-decimal IP addresses because they simply have no other way for
one host to refer to the other by name. This is a sorry state of
affairs. What is worse, most users don't even bother trying to use
dotted-decimal IP addresses. Most users still move data between
machines by burning it onto CD-R, copying it onto a USB "keychain"
flash drive, or similar removable media.
In a world of gigabit Ethernet and ubiquitous wireless networking, it
is a sad indictment of the networking community that most users still
prefer sneakernet.
Allowing ad hoc allocation of single-label names in a single flat
".local." namespace may seem to invite chaos. However, operational
experience with AppleTalk NBP names [ATalk], which on any given link
are also effectively single-label names in a flat namespace, shows
that in practice name collisions happen extremely rarely and are not
a problem. Groups of computer users from disparate organizations
bring Macintosh laptop computers to events such as IETF Meetings, the
Mac Hack conference, the Apple World Wide Developer Conference, etc.,
and complaints at these events about users suffering conflicts and
being forced to rename their machines have never been an issue.
This document recommends a single flat namespace for dot-local host
names, (i.e. the names of DNS "A" and "AAAA" records, which map names
to IPv4 and IPv6 addresses), but other DNS record types (such as
those used by DNS Service Discovery [DNS-SD]) may contain as many
labels as appropriate for the desired usage, subject to the 256-byte
name length limit specified below in Section 3.3 "Maximum Multicast
DNS Name Length".
Enforcing uniqueness of host names is probably desirable in the
common case, but this document does not mandate that. It is
permissible for a collection of coordinated hosts to agree to
maintain multiple DNS address records with the same name, possibly
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for load balancing or fault-tolerance reasons. This document does not
take a position on whether that is sensible. It is important that
both modes of operation are supported. The Multicast DNS protocol
allows hosts to verify and maintain unique names for resource records
where that behavior is desired, and it also allows hosts to maintain
multiple resource records with a single shared name where that
behavior is desired. This consideration applies to all resource
records, not just address records (host names). In summary: It is
required that the protocol have the ability to detect and handle name
conflicts, but it is not required that this ability be used for every
record.
3.1 Governing Standards Body
Note that this use of the ".local." suffix falls under IETF/IANA
jurisdiction, not ICANN jurisdiction. DNS is an IETF network
protocol, governed by protocol rules defined by the IETF. These IETF
protocol rules dictate character set, maximum name length, packet
format, etc. ICANN determines additional rules that apply when the
IETF's DNS protocol is used on the public Internet. In contrast,
private uses of the DNS protocol on isolated private networks are not
governed by ICANN. Since this change is a change to the core DNS
protocol rules, it affects everyone, not just those machines using
the ICANN-governed Internet. Hence this change falls into the
category of an IETF protocol rule, not an ICANN usage rule.
This allocation of responsibility is formally established in
"Memorandum of Understanding Concerning the Technical Work of the
Internet Assigned Numbers Authority" [RFC 2860]. Exception (a) of
clause 4.3 states that the IETF has the authority to instruct IANA
to reserve pseudo-TLDs as required for protocol design purposes.
For example, "Reserved Top Level DNS Names" [RFC 2606] defines
the following pseudo-TLDs:
.test
.example
.invalid
.localhost
3.2 Private DNS Namespaces
Note also that the special treatment of names ending in ".local." has
been implemented in Macintosh computers since the days of Mac OS 9,
and continues today in Mac OS X. There are also implementations for
Microsoft Windows [B4W], Linux and other platforms. Operators setting
up private internal networks ("intranets") are advised that their
lives may be easier if they avoid using the suffix ".local." in names
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in their private internal DNS server. Alternative possibilities
include:
.intranet
.internal
.private
.corp
.home
.lan
Another alternative naming scheme, advocated by Professor D. J.
Bernstein, is to use a numerical suffix, such as ".6." [djbdl].
3.3 Maximum Multicast DNS Name Length
RFC 1034 says:
the total number of octets that represent a domain name (i.e.,
the sum of all label octets and label lengths) is limited to 255.
This text does not state whether the final root label at the end of
every name should be included in this count. However, "Clarifications
to the DNS Specification" [RFC 2181] does offer one clue:
The zero length full name is defined as representing the root
of the DNS tree, and is typically written and displayed as ".".
If the empty root label, represented in the packet by a single zero
byte, and typically written and displayed as ".", is defined to be
the "zero length name", then for consistency, the final root label
(zero byte) in all names should be similarly ignored. This yields
the following nominal length (NL) calculations:
--------
| 0x00 | NL = 0
--------
---------------------------
| 0x03 | c | o | m | 0x00 | NL = 4
---------------------------
------------------------------------------------------
| 0x05 | a | p | p | l | e | 0x03 | c | o | m | 0x00 | NL = 10
------------------------------------------------------
This means that the maximum length of a domain name, as represented
in a Multicast DNS packet, MUST NOT exceed 255 bytes *excluding*
the final terminating zero, or 256 bytes *including* the final
terminating zero.
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4. Source Address Check
All Multicast DNS responses (including responses sent via unicast)
SHOULD be sent with IP TTL set to 255. This is recommended to provide
backwards-compatibility with older Multicast DNS clients that check
the IP TTL on reception to determine whether the packet originated
on the local link. These older clients discard all packets with TTLs
other than 255.
A host sending Multicast DNS queries to a link-local destination
address (including the 224.0.0.251 link-local multicast address)
MUST only accept responses to that query that originate from the
local link, and silently discard any other response packets. Without
this check, it could be possible for remote rogue hosts to send
spoof answer packets (perhaps unicast to the victim host) which the
receiving machine could misinterpret as having originated on the
local link.
The test for whether a response originated on the local link
is done in two ways:
* All responses sent to the link-local multicast address 224.0.0.251
are necessarily deemed to have originated on the local link,
regardless of source IP address. This is essential to allow devices
to work correctly and reliably in unusual configurations, such as
multiple logical IP subnets overlayed on a single link, or in cases
of severe misconfiguration, where devices are physically connected
to the same link, but are currently misconfigured with completely
unrelated IP addresses and subnet masks.
* For responses sent to a unicast destination address, the source IP
address in the packet is checked to see if it is an address on a
local subnet. An address is determined to be on a local subnet if,
for (one of) the address(es) configured on the interface receiving
the packet, (I & M) == (P & M), where I and M are the interface
address and subnet mask respectively, P is the source IP address
from the packet, '&' represents the bitwise logical 'and'
operation, and '==' represents a bitwise equality test.
Since queriers will ignore responses apparently originating outside
the local subnet, a Responder SHOULD avoid generating responses that
it can reasonably predict will be ignored. This applies particularly
in the case of overlayed subnets. If a Responder receives a query
addressed to the link-local multicast address 224.0.0.251, from a
source address not apparently on the same subnet as the Responder,
then even if the query indicates that a unicast response is preferred
(see Section 6.5, "Questions Requesting Unicast Responses"), the
Responder SHOULD elect to respond by multicast anyway, since it can
reasonably predict that a unicast response with an apparently
non-local source address will probably be ignored.
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5. Reverse Address Mapping
Like ".local.", the IPv4 and IPv6 reverse mapping domains are also
defined to be link-local:
Any DNS query for a name ending with "254.169.in-addr.arpa." MUST
be sent to the mDNS multicast address 224.0.0.251. Since names
under this domain correspond to IPv4 link-local addresses, it is
logical that the local link is the best place to find information
pertaining to those names.
Likewise, any DNS query for a name within the reverse mapping
domains for IPv6 Link-Local addresses ("8.e.f.ip6.arpa.",
"9.e.f.ip6.arpa.", "a.e.f.ip6.arpa.", and "b.e.f.ip6.arpa.") MUST
be sent to the IPv6 mDNS link-local multicast address FF02::FB.
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6. Querying
There are three kinds of Multicast DNS Queries, one-shot queries of
the kind made by today's conventional DNS clients, one-shot queries
accumulating multiple responses made by multicast-aware DNS clients,
and continuous ongoing Multicast DNS Queries used by IP network
browser software.
A Multicast DNS Responder that is offering records that are intended
to be unique on the local link MUST also implement a Multicast DNS
Querier so that it can first verify the uniqueness of those records
before it begins answering queries for them.
6.1 One-Shot Multicast DNS Queries
The most basic kind of Multicast DNS client may simply send its DNS
queries blindly to 224.0.0.251:5353, without necessarily even being
aware of what a multicast address is. This change can typically be
implemented with just a few lines of code in an existing DNS resolver
library. Any time the name being queried for falls within one of the
reserved mDNS domains (see Section 12 "Special Characteristics of
Multicast DNS Domains") the query is sent to 224.0.0.251:5353 instead
of the configured unicast DNS server address that would otherwise be
used. Typically the timeout would also be shortened to two or three
seconds. It's possible to make a minimal mDNS client with only these
simple changes.
A simple DNS client like this will typically just take the first
response it receives. It will not listen for additional UDP
responses, but in many instances this may not be a serious problem.
If a user types "http://cheshire.local." into their Web browser and
gets to see the page they were hoping for, then the protocol has met
the user's needs in this case.
While a basic DNS client like this may be adequate for simple
hostname lookup, it may not get ideal behavior in other cases.
Additional refinements that may be adopted by more sophisticated
clients are described below.
6.2 One-Shot Queries, Accumulating Multiple Responses
A more sophisticated DNS client should understand that Multicast DNS
is not exactly the same as unicast DNS, and should modify its
behavior in some simple ways.
As described above, there are some cases, such as looking up the
address associated with a unique host name, where a single response
is sufficient, and moreover may be all that is expected. However,
there are other DNS queries where more than one response is
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possible, and for these queries a more advanced Multicast DNS client
should include the ability to wait for an appropriate period of time
to collect multiple responses.
A naive DNS client retransmits its query only so long as it has
received no response. A more advanced Multicast DNS client is aware
that having received one response is not necessarily an indication
that it might not receive others, and has the ability to retransmit
its query until it is satisfied with the collection of responses it
has gathered. When retransmitting, the interval between the first two
queries SHOULD be at least one second, and the intervals between
successive queries SHOULD increase by at least a factor of two.
A Multicast DNS client that is retransmitting a query for which it
has already received some responses MUST implement Known Answer
Suppression, as described below in Section 7.1 "Known Answer
Suppression". This indicates to Responders who have already replied
that their responses have been received, and they don't need to send
them again in response to this repeated query.
6.3 Continuous Multicast DNS Querying
In One-Shot Queries, with either single or multiple responses,
the underlying assumption is that the transaction begins when the
application issues a query, and ends when the desired responses
have been received. There is another type of operation which is more
akin to continuous monitoring.
iTunes users are accustomed to seeing a list of shared network music
libraries in the sidebar of the iTunes window. There is no "refresh"
button for the user to click because the list is expected to be
always accurate, always reflecting the currently available libraries,
without the user having to take any manual action to keep it that
way. When a new library becomes available it promptly appears in the
list, and when a library becomes unavailable it promptly disappears.
It is vitally important that this responsive user interface be
achieved without naive polling that would place an unreasonable
burden on the network.
Therefore, when retransmitting mDNS queries to implement this kind of
continuous monitoring, the interval between the first two queries
SHOULD be at least one second, the intervals between successive
queries SHOULD increase by at least a factor of two, and the querier
MUST implement Known Answer Suppression, as described below in
Section 7.1. When the interval between queries reaches or exceeds 60
minutes, a querier MAY cap the interval to a maximum of 60 minutes,
and perform subsequent queries at a steady-state rate of one query
per hour. To avoid accidental synchronization when for some reason
multiple clients begin querying at exactly the same moment (e.g.
because of some common external trigger event), a Multicast DNS
Querier SHOULD also delay the first query of the series by a
randomly-chosen amount in the range 20-120ms.
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When a Multicast DNS Querier receives an answer, the answer contains
a TTL value that indicates for how many seconds this answer is valid.
After this interval has passed, the answer will no longer be valid
and SHOULD be deleted from the cache. Before this time is reached,
a Multicast DNS Querier which has clients with an active interest in
the state of that record (e.g. a network browsing window displaying
a list of discovered services to the user) SHOULD re-issue its query
to determine whether the record is still valid.
To perform this cache maintenance, a Multicast DNS Querier should
plan to re-query for records after at least 50% of the record
lifetime has elapsed. This document recommends the following
specific strategy:
The Querier should plan to issue a query at 80% of the record
lifetime, and then if no answer is received, at 85%, 90% and 95%.
If an answer is received, then the remaining TTL is reset to the
value given in the answer, and this process repeats for as long as
the Multicast DNS Querier has an ongoing interest in the record.
If after four queries no answer is received, the record is deleted
when it reaches 100% of its lifetime. A Multicast DNS Querier MUST
NOT perform this cache maintenance for records for which it has no
clients with an active interest. If the expiry of a particular record
from the cache would result in no net effect to any client software
running on the Querier device, and no visible effect to the human
user, then there is no reason for the Multicast DNS Querier to
waste network bandwidth checking whether the record remains valid.
To avoid the case where multiple Multicast DNS Queriers on a network
all issue their queries simultaneously, a random variation of 2% of
the record TTL should be added, so that queries are scheduled to be
performed at 80-82%, 85-87%, 90-92% and then 95-97% of the TTL.
An additional efficiency optimization SHOULD be performed when
a Multicast DNS response is received containing a unique answer
(as indicated by the cache flush bit being set (see Section 11.3,
"Announcements to Flush Outdated Cache Entries"). In this case, there
is no need for the querier to continue issuing a stream of queries
with exponentially-increasing intervals, since the receipt of a
unique answer is a good indication that no other answers will be
forthcoming. In this case, the Multicast DNS Querier SHOULD plan to
issue its next query for this record at 80-82% of the record's TTL,
as described above.
6.4 Multiple Questions per Query
Multicast DNS allows a querier to place multiple questions in the
Question Section of a single Multicast DNS query packet.
The semantics of a Multicast DNS query packet containing multiple
questions is identical to a series of individual DNS query packets
containing one question each. Combining multiple questions into a
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single packet is purely an efficiency optimization, and has no other
semantic significance.
6.5 Questions Requesting Unicast Responses
Sending Multicast DNS responses via multicast has the benefit that
all the other hosts on the network get to see those responses, and
can keep their caches up to date, and can detect conflicting
responses.
However, there are situations where all the other hosts on the
network don't need to see every response. Some examples are a laptop
computer waking from sleep, or the Ethernet cable being connected to
a running machine, or a previously inactive interface being activated
through a configuration change. At the instant of wake-up or link
activation, the machine is a brand new participant on a new network.
Its Multicast DNS cache for that interface is empty, and it has no
knowledge of its peers on that link. It may have a significant number
of questions that it wants answered right away to discover
information about its new surroundings and present that information
to the user. As a new participant on the network, it has no idea
whether the exact same questions may have been asked and answered
just seconds ago. In this case, triggering a large sudden flood of
multicast responses may impose an unreasonable burden on the network.
To avoid large floods of potentially unnecessary responses in these
cases, Multicast DNS defines the top bit in the class field of a DNS
question as the "unicast response" bit. When this bit is set in a
question, it indicates that the Querier is willing to accept unicast
responses instead of the usual multicast responses. These questions
requesting unicast responses are referred to as "QU" questions, to
distinguish them from the more usual questions requesting multicast
responses ("QM" questions). A Multicast DNS Querier sending its
initial batch of questions immediately on wake from sleep or
interface activation SHOULD set the "QU" bit in those questions.
When a question is retransmitted (as described in Section 6.3
"Continuous Multicast DNS Querying") the "QU" bit SHOULD NOT be set
in subsequent retransmissions of that question. Subsequent
retransmissions SHOULD be usual "QM" questions. After the first
question has received its responses, the querier should have a large
known-answer list (see "Known Answer Suppression" below) so that
subsequent queries should elicit few, if any, further responses.
Reverting to multicast responses as soon as possible is important
because of the benefits that multicast responses provide (see
"Benefits of Multicast Responses" below). In addition, the "QU" bit
SHOULD be set only for questions that are active and ready to be sent
the moment of wake from sleep or interface activation. New questions
issued by clients afterwards should be treated as normal "QM"
questions and SHOULD NOT have the "QU" bit set on the first question
of the series.
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When receiving a question with the "unicast response" bit set, a
Responder SHOULD usually respond with a unicast packet directed back
to the querier. If the Responder has not multicast that record
recently (within one quarter of its TTL), then the Responder SHOULD
instead multicast the response so as to keep all the peer caches up
to date, and to permit passive conflict detection. In the case of
answering a probe question with the "unicast response" bit set, the
Responder should always generate the requested unicast response, but
may also send a multicast announcement too if the time since the last
multicast announcement of that record is more than a quarter of its
TTL.
Except when defending a unique name against a probe from another
host, unicast replies are subject to all the same packet generation
rules as multicast replies, including the cache flush bit (see
Section 11.3, "Announcements to Flush Outdated Cache Entries") and
randomized delays to reduce network collisions (see Section 8,
"Responding").
6.6 Delaying Initial Query
If a query is issued for which there already exist one or more
records in the local cache, and those record(s) were received with
the cache flush bit set (see Section 11.3, "Announcements to Flush
Outdated Cache Entries"), indicating that they form a unique RRSet,
then the host SHOULD delay its initial query by imposing a random
delay from 500-1000ms. This is to avoid the situation where a group
of hosts are synchronized by some external event and all perform
the same query simultaneously. This means that when the first host
(selected randomly by this algorithm) transmits its query, all the
other hosts that were about to transmit the same query can suppress
their superfluous queries, as described in "Duplicate Question
Suppression" below.
6.7 Direct Unicast Queries to port 5353
In specialized applications there may be rare situations where it
makes sense for a Multicast DNS Querier to send its query via unicast
to a specific machine. When a Multicast DNS Responder receives a
query via direct unicast, it SHOULD respond as it would for a
"QU" query, as described above in Section 6.5 "Questions Requesting
Unicast Responses". Since it is possible for a unicast query to be
received from a machine outside the local link, Responders SHOULD
check that the source address in the query packet matches the local
subnet for that link, and silently ignore the packet if not.
There may be specialized situations, outside the scope of this
document, where it is intended and desirable to create a Responder
that does answer queries originating outside the local link. Such
a Responder would need to ensure that these non-local queries are
always answered via unicast back to the Querier, since an answer sent
via link-local multicast would not reach a Querier outside the local
link.
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7. Duplicate Suppression
A variety of techniques are used to reduce the amount of redundant
traffic on the network.
7.1 Known Answer Suppression
When a Multicast DNS Querier sends a query to which it already knows
some answers, it populates the Answer Section of the DNS message with
those answers.
A Multicast DNS Responder MUST NOT answer a Multicast DNS Query if
the answer it would give is already included in the Answer Section
with an RR TTL at least half the correct value. If the RR TTL of the
answer as given in the Answer Section is less than half of the true
RR TTL as known by the Multicast DNS Responder, the Responder MUST
send an answer so as to update the Querier's cache before the record
becomes in danger of expiration.
Because a Multicast DNS Responder will respond if the remaining TTL
given in the known answer list is less than half the true TTL, it is
superfluous for the Querier to include such records in the known
answer list. Therefore a Multicast DNS Querier SHOULD NOT include
records in the known answer list whose remaining TTL is less than
half their original TTL. Doing so would simply consume space in the
packet without achieving the goal of suppressing responses, and would
therefore be a pointless waste of network bandwidth.
A Multicast DNS Querier MUST NOT cache resource records observed in
the Known Answer Section of other Multicast DNS Queries. The Answer
Section of Multicast DNS Queries is not authoritative. By placing
information in the Answer Section of a Multicast DNS Query the
querier is stating that it *believes* the information to be true.
It is not asserting that the information *is* true. Some of those
records may have come from other hosts that are no longer on the
network. Propagating that stale information to other Multicast DNS
Queriers on the network would not be helpful.
7.2 Multi-Packet Known Answer Suppression
Sometimes a Multicast DNS Querier will already have too many answers
to fit in the Known Answer Section of its query packets. In this
case, it should issue a Multicast DNS Query containing a question and
as many Known Answer records as will fit. It MUST then set the TC
(Truncated) bit in the header before sending the Query. It MUST then
immediately follow the packet with another query packet containing no
questions, and as many more Known Answer records as will fit. If
there are still too many records remaining to fit in the packet, it
again sets the TC bit and continues until all the Known Answer
records have been sent.
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A Multicast DNS Responder seeing a Multicast DNS Query with the TC
bit set defers its response for a time period randomly selected in
the interval 400-500ms. This gives the Multicast DNS Querier time to
send additional Known Answer packets before the Responder responds.
If the Responder sees any of its answers listed in the Known Answer
lists of subsequent packets from the querying host, it SHOULD delete
that answer from the list of answers it is planning to give, provided
that no other host on the network is also waiting to receive the same
answer record.
If the Responder receives additional Known Answer packets with the TC
bit set, it SHOULD extend the delay as necessary to ensure a pause of
400-500ms after the last such packet before it sends its answer. This
opens the potential risk that a continuous stream of Known Answer
packets could, theoretically, prevent a Responder from answering
indefinitely. In practice answers are never actually delayed
significantly, and should a situation arise where significant delays
did happen, that would be a scenario where the network is so
overloaded that it would be desirable to err on the side of caution.
The consequence of delaying an answer may be that it takes a user
longer than usual to discover all the services on the local network;
in contrast the consequence of incorrectly answering before all the
Known Answer packets have been received would be wasting bandwidth
sending unnecessary answers on an already overloaded network. In this
(rare) situation, sacrificing speed to preserve reliable network
operation is the right trade-off.
7.3 Duplicate Question Suppression
If a host is planning to send a query, and it sees another host on
the network send a QM query containing the same question, and the
Known Answer Section of that query does not contain any records which
this host would not also put in its own Known Answer Section, then
this host should treat its own query as having been sent. When
multiple clients on the network are querying for the same resource
records, there is no need for them to all be repeatedly asking the
same question.
7.4 Duplicate Answer Suppression
If a host is planning to send an answer, and it sees another host on
the network send a response packet containing the same answer record,
and the TTL in that record is not less than the TTL this host would
have given, then this host should treat its own answer as having been
sent. When multiple Responders on the network have the same data,
there is no need for all of them to respond.
This feature is particularly useful when multiple Sleep Proxy Servers
are deployed (see Section 17, "Multicast DNS and Power Management").
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In the future it is possible that every general-purpose OS (Mac,
Windows, Linux, etc.) will implement Sleep Proxy Service as a matter
of course. In this case there could be a large number of Sleep Proxy
Servers on any given network, which is good for reliability and
fault-tolerance, but would be bad for the network if every Sleep
Proxy Server were to answer every query.
8. Responding
When a Multicast DNS Responder constructs and sends a Multicast DNS
response packet, the Answer Section of that packet must contain only
records for which that Responder is explicitly authoritative. These
answers may be generated because the record answers a question
received in a Multicast DNS query packet, or at certain other times
that the Responder determines than an unsolicited announcement is
warranted. A Multicast DNS Responder MUST NOT place records from its
cache, which have been learned from other Responders on the network,
in the Answer Section of outgoing response packets. Only an
authoritative source for a given record is allowed to issue responses
containing that record.
The determination of whether a given record answers a given question
is done using the standard DNS rules: The record name must match the
question name, the record rrtype must match the question qtype
(unless the qtype is "ANY"), and the record rrclass must match the
question qclass (unless the qclass is "ANY").
A Multicast DNS Responder MUST only respond when it has a positive
non-null response to send, or it authoritatively knows that a
particular record does not exist. For unique records, where the host
has already established sole ownership of the name, it MUST return
negative answers to queries for records that it knows not to exist.
For example, a host with no IPv6 address, that has claimed sole
ownership of the name "host.local." for all rrtypes, MUST respond to
AAAA queries for "host.local." by sending a negative answer
indicating that no AAAA records exist for that name. See Section 8.1
"Negative Responses". For shared records, which are owned by no
single host, the nonexistence of a given record is ascertained by the
failure of any machine to respond to the Multicast DNS query, not by
any explicit negative response. NXDOMAIN and other error responses
must not be sent.
Multicast DNS Responses MUST NOT contain any questions in the
Question Section. Any questions in the Question Section of a received
Multicast DNS Response MUST be silently ignored. Multicast DNS
Queriers receiving Multicast DNS Responses do not care what question
elicited the response; they care only that the information in the
response is true and accurate.
A Multicast DNS Responder on Ethernet [IEEE 802] and similar shared
multiple access networks SHOULD have the capability of delaying its
responses by up to 500ms, as determined by the rules described below.
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If a large number of Multicast DNS Responders were all to respond
immediately to a particular query, a collision would be virtually
guaranteed. By imposing a small random delay, the number of
collisions is dramatically reduced. On a full-sized Ethernet using
the maximum cable lengths allowed and the maximum number of repeaters
allowed, an Ethernet frame is vulnerable to collisions during the
transmission of its first 256 bits. On 10Mb/s Ethernet, this equates
to a vulnerable time window of 25.6us. On higher-speed variants of
Ethernet, the vulnerable time window is shorter.
In the case where a Multicast DNS Responder has good reason to
believe that it will be the only Responder on the link that will send
a response (i.e. because it is able to answer every question in the
query packet, and for all of those answer records it has previously
verified that the name, rrtype and rrclass are unique on the link)
it SHOULD NOT impose any random delay before responding, and SHOULD
normally generate its response within at most 10ms. In particular,
this applies to responding to probe queries with the "unicast
response" bit set. Since receiving a probe query gives a clear
indication that some other Responder is planning to start using this
name in the very near future, answering such probe queries to defend
a unique record is a high priority and needs to be done immediately,
without delay. A probe query can be distinguished from a normal query
by the fact that a probe query contains a proposed record in the
Authority Section which answers the question in the Question Section
(for more details, see Section 9.1, "Probing").
Responding immediately without delay is appropriate for records like
the address record for a particular host name, when the host name has
been previously verified unique. Responding immediately without delay
is *not* appropriate for things like looking up PTR records used for
DNS Service Discovery [DNS-SD], where a large number of responses may
be anticipated.
In any case where there may be multiple responses, such as queries
where the answer is a member of a shared resource record set, each
Responder SHOULD delay its response by a random amount of time
selected with uniform random distribution in the range 20-120ms.
The reason for requiring that the delay be at least 20ms is to
accommodate the situation where two or more query packets are sent
back-to-back, because in that case we want a Responder with answers
to more than one of those queries to have the opportunity to
aggregate all of its answers into a single response packet.
In the case where the query has the TC (truncated) bit set,
indicating that subsequent known answer packets will follow,
Responders SHOULD delay their responses by a random amount of time
selected with uniform random distribution in the range 400-500ms,
to allow enough time for all the known answer packets to arrive,
as described in Section 7.2 "Multi-Packet Known Answer Suppression".
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Except when a unicast response has been explicitly requested (via the
"unicast response" bit, by virtue of being a Legacy Query (Section
8.5), or by virtue of being a direct unicast query) Multicast DNS
Responses MUST be sent to UDP port 5353 (the well-known port assigned
to mDNS) on the 224.0.0.251 multicast address (or its IPv6 equivalent
FF02::FB). Operating in a Zeroconf environment requires constant
vigilance. Just because a name has been previously verified unique
does not mean it will continue to be so indefinitely. By allowing all
Multicast DNS Responders to constantly monitor their peers'
responses, conflicts arising out of network topology changes can be
promptly detected and resolved. Sending all responses by multicast
also facilitates opportunistic caching by other hosts on the network.
To protect the network against excessive packet flooding due to
software bugs or malicious attack, a Multicast DNS Responder MUST NOT
(except in the one special case of answering probe queries) multicast
a record on a given interface until at least one second has elapsed
since the last time that record was multicast on that particular
interface. A legitimate client on the network should have seen the
previous transmission and cached it. A client that did not receive
and cache the previous transmission will retry its request and
receive a subsequent response. In the special case of answering probe
queries, because of the limited time before the probing host will
make its decision about whether or not to use the name, a Multicast
DNS Responder MUST respond quickly. In this special case only, when
responding via multicast to a probe, a Multicast DNS Responder is
only required to delay its transmission as necessary to ensure an
interval of at least 250ms since the last time the record was
multicast on that interface.
8.1 Negative Responses
In the early design of Multicast DNS it was assumed that explicit
negative responses would never be needed. Hosts can assert the
existence of records which the host claims to exist, but attempting
the converse -- asserting the non-existence of all possible Multicast
DNS records that could exist on this network but do not at this
moment -- was felt to be impractical. The non-existence of a record
would be ascertained by querying for it and failing to receive any
responses.
However, operational experience showed that explicit negative
responses are important in one case in particular -- clients querying
for a AAAA record when the host in question has no IPv6 addresses.
In this case the host knows it currently has exclusive ownership of
that name, and the host knows it currently does not have any IPv6
addresses, so an explicit negative response is preferable to the
client having to retransmit its query multiple times and eventually
give up with a timeout before it can conclude that a given AAAA
record does not exist.
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A Multicast DNS Responder indicates the nonexistence of a record by
using a DNS NSEC record [RFC 3845]. In the case of Multicast DNS
the NSEC record is not being used for its usual DNSSEC security
properties, but simply as a way of expressing which records do or do
not exist with a given name. As such, a restricted form of the NSEC
record is used with Multicast DNS:
o The 'Next Domain Name' field always contains the record's own
name. When used with name compression, this means that the 'Next
Domain Name' field always takes exactly two bytes in the packet.
o The Type Bit Map block number is always 0.
o The Type Bit Map block length byte is a value in the range 1-32.
o The Type Bit Map data is 1-32 bytes, as indicated by length byte.
Because a Multicast DNS NSEC record is limited to Type Bit Map block
number zero, it cannot express the existence of rrtypes above 255.
Because of this, if a Multicast DNS Responder were to have records
with rrtypes above 255, it MUST NOT generate Multicast DNS NSEC
records for those names, since to do so would imply that the name
has no records with rrtypes above 255, which would be incorrect.
In practice this is not a significant limitation, since rrtypes
above 255 are not currently in widespread use.
If a Multicast DNS implementation receives an NSEC record where the
'Next Domain Name' field is not the record's own name, then the
implementation MUST ignore the 'Next Domain Name' field and process
the NSEC record as usual. In Multicast DNS the 'Next Domain Name'
field is not currently used.
If a Multicast DNS implementation receives an NSEC record where
the Type Bit Map block number is not zero, or the block length
is not in the range 1-32, then the entire NSEC record MUST be
silently ignored.
To help differentiate these synthesized NSEC records (generated
programmatically on-the-fly) from conventional Unicast DNS NSEC
records (which actually exist in a signed DNS zone) the synthesized
Multicast DNS NSEC records MUST NOT have the 'NSEC' bit set in the
Type Bit Map, whereas conventional Unicast DNS NSEC records do have
the 'NSEC' bit set.
The TTL of the NSEC record indicates the intended lifetime of the
negative cache entry. In general, the TTL given for an NSEC record
SHOULD be the same as the TTL that the record would have had, had it
existed. For example, the TTL for address records in Multicast DNS is
typically 120 seconds, so the negative cache lifetime for an address
record that does not exist should also be 120 seconds.
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A Responder should only generate negative responses to queries for
which it has legitimate ownership of the name/rrtype/rrclass in
question, and can legitimately assert that no record with that
name/rrtype/rrclass exists. A Responder can assert that a specified
rrtype does not exist for one of its names only if it previously
claimed unique ownership of that name using probe queries for rrtype
ANY. (If it were to use probe queries for a specific rrtype, then it
would only own the name for that rrtype, and could not assert that
other rrtypes do not exist.) Similarly, a Responder can assert that a
specified rrclass does not exist for one of its names only if it
previously claimed unique ownership of that name using probe queries
for rrclass ANY. On receipt of a question for a particular
name/rrtype/rrclass which a Responder knows not to exist by virtue of
previous successful probing, the Responder MUST send a response
packet containing the appropriate NSEC record.
The obvious solution of using an NXDOMAIN response does not apply
well for Multicast DNS. A Unicast DNS NXDOMAIN response applies to
the entire packet, but for efficiency Multicast DNS tries to pack
multiple responses into a packet. If the error code in the header
were NXDOMAIN, it would not be clear to which record(s) that error
code applied.
A benefit of asserting nonexistence through NSEC records instead of
through NXDOMAIN responses is that NSEC records can be added to the
Additional Section of a DNS Response to offer additional information
beyond what the client explicitly requested. For example, in a
response to an SRV query, a Responder SHOULD include 'A' record(s)
giving its IPv4 addresses in the Additional Section, and if it has no
IPv6 addresses then it SHOULD include an NSEC record indicating this
fact in the Additional Section too. In effect, the Responder is
saying, "Here's my SRV record, and here are my IPv4 addresses, and
no, I don't have any IPv6 addresses, so don't waste your time
asking." Without this information in the Additional Section it would
take the client an additional round-trip to perform an additional
Query to ascertain that the target host has no AAAA records.
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8.2 Responding to Address Queries
In Multicast DNS, whenever a Responder places an IPv4 or IPv6 address
record (rrtype "A" or "AAAA") into a response packet, it SHOULD also
place the corresponding other address type into the additional
section, if there is space in the packet.
This is to provide fate sharing, so that all a device's addresses are
delivered atomically in a single packet, to reduce the risk that
packet loss could cause a querier to receive only the IPv4 addresses
and not the IPv6 addresses, or vice versa.
In the event that a device has only IPv4 addresses but no IPv6
addresses, or vice versa, then the appropriate NSEC record SHOULD
be placed into the additional section, so that queriers can know
with certainty that the device has no addresses of that kind.
Some Multicast DNS Responders treat a physical interface with both
IPv4 and IPv6 address as a single interface with two addresses. Other
Multicast DNS Responders treat this case as logically two interfaces,
each with one address, but Responders that operate this way MUST NOT
put the corresponding automatic NSEC records in replies they send
(i.e. a negative IPv4 assertion in their IPv6 responses, and a
negative IPv6 assertion in their IPv4 responses) because this would
cause incorrect operation in Responders on the network that work the
former way.
8.3 Responding to Multi-Question Queries
Multicast DNS Responders MUST correctly handle DNS query packets
containing more than one question, by answering any or all of the
questions to which they have answers. Any (non-defensive) answers
generated in response to query packets containing more than one
question SHOULD be randomly delayed in the range 20-120ms, or
400-500ms if the TC (truncated) bit is set, as described above.
(Answers defending a name, in response to a probe for that name,
are not subject to this delay rule and are still sent immediately.)
8.4 Response Aggregation
When possible, a Responder SHOULD, for the sake of network
efficiency, aggregate as many responses as possible into a single
Multicast DNS response packet. For example, when a Responder has
several responses it plans to send, each delayed by a different
interval, then earlier responses SHOULD be delayed by up to an
additional 500ms if that will permit them to be aggregated with
other responses scheduled to go out a little later.
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8.5 Legacy Unicast Responses
If the source UDP port in a received Multicast DNS Query is not port
5353, this indicates that the client originating the query is a
simple client that does not fully implement all of Multicast DNS.
In this case, the Multicast DNS Responder MUST send a UDP response
directly back to the client, via unicast, to the query packet's
source IP address and port. This unicast response MUST be a
conventional unicast response as would be generated by a conventional
unicast DNS server; for example, it MUST repeat the query ID and the
question given in the query packet.
The resource record TTL given in a legacy unicast response SHOULD NOT
be greater than ten seconds, even if the true TTL of the Multicast
DNS resource record is higher. This is because Multicast DNS
Responders that fully participate in the protocol use the cache
coherency mechanisms described in Section 11 "Resource Record TTL
Values and Cache Coherency" to update and invalidate stale data. Were
unicast responses sent to legacy clients to use the same high TTLs,
these legacy clients, which do not implement these cache coherency
mechanisms, could retain stale cached resource record data long after
it is no longer valid.
Having sent this unicast response, if the Responder has not sent this
record in any multicast response recently, it SHOULD schedule the
record to be sent via multicast as well, to facilitate passive
conflict detection. "Recently" in this context means "if the time
since the record was last sent via multicast is less than one quarter
of the record's TTL".
Note that while legacy queries usually contain exactly one question,
they are permitted to contain multiple questions, and Responders
listening for multicast queries on 224.0.0.251:5353 MUST be prepared
to handle this correctly, responding by generating a unicast response
containing the list of question(s) they are answering in the Question
Section, and the records answering those question(s) in the Answer
Section.
9. Probing and Announcing on Startup
Typically a Multicast DNS Responder should have, at the very least,
address records for all of its active interfaces. Creating and
advertising an HINFO record on each interface as well can be useful
to network administrators.
Whenever a Multicast DNS Responder starts up, wakes up from sleep,
receives an indication of an Ethernet "Link Change" event, or has any
other reason to believe that its network connectivity may have
changed in some relevant way, it MUST perform the two startup steps
below: Probing (Section 9.1) and Announcing (Section 9.3).
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9.1 Probing
The first startup step is that for all those resource records that a
Multicast DNS Responder desires to be unique on the local link, it
MUST send a Multicast DNS Query asking for those resource records, to
see if any of them are already in use. The primary example of this is
its address records which map its unique host name to its unique IPv4
and/or IPv6 addresses. All Probe Queries SHOULD be done using the
desired resource record name and query type ANY (255), to elicit
answers for all types of records with that name. This allows a single
question to be used in place of several questions, which is more
efficient on the network. It also allows a host to verify exclusive
ownership of a name for all rrtypes, which is desirable in most
cases. It would be confusing, for example, if one host owned the "A"
record for "myhost.local.", but a different host owned the HINFO
record for that name.
The ability to place more than one question in a Multicast DNS Query
is useful here, because it can allow a host to use a single packet
for all of its resource records instead of needing a separate packet
for each. For example, a host can simultaneously probe for uniqueness
of its "A" record and all its SRV records [DNS-SD] in the same query
packet.
When ready to send its mDNS probe packet(s) the host should first
wait for a short random delay time, uniformly distributed in the
range 0-250ms. This random delay is to guard against the case where a
group of devices are powered on simultaneously, or a group of devices
are connected to an Ethernet hub which is then powered on, or some
other external event happens that might cause a group of hosts to all
send synchronized probes.
250ms after the first query the host should send a second, then
250ms after that a third. If, by 250ms after the third probe, no
conflicting Multicast DNS responses have been received, the host may
move to the next step, announcing. (Note that this is the one
exception from the normal rule that there should be at least one
second between repetitions of the same question, and the interval
between subsequent repetitions should at least double.)
When sending probe queries, a host MUST NOT consult its cache for
potential answers. Only conflicting Multicast DNS responses received
"live" from the network are considered valid for the purposes of
determining whether probing has succeeded or failed.
In order to allow services to announce their presence without
unreasonable delay, the time window for probing is intentionally set
quite short. As a result of this, from the time the first probe
packet is sent, another device on the network using that name has
just 750ms to respond to defend its name. On networks that are slow,
or busy, or both, it is possible for round-trip latency to account
for a few hundred milliseconds, and software delays in slow devices
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can add additional delay. For this reason, it is important that when
a device receives a probe query for a name that it is currently using
for unique records, it SHOULD generate its response to defend that
name immediately and send it as quickly as possible. The usual rules
about random delays before responding, to avoid sudden bursts of
simultaneous answers from different hosts, do not apply here since
at most one host should ever respond to a given probe question. Even
when a single DNS query packet contains multiple probe questions,
it would be unusual for that packet to elicit a defensive response
from more than one other host. Because of the mDNS multicast rate
limiting rules, the first two probes SHOULD be sent as "QU" questions
with the "unicast response" bit set, to allow a defending host to
respond immediately via unicast, instead of potentially having to
wait before replying via multicast. At the present time, this
document recommends that the third probe SHOULD be sent as a standard
"QM" question, for backwards compatibility with the small number of
old devices still in use that don't implement unicast responses.
If, at any time during probing, from the beginning of the initial
random 0-250ms delay onward, any conflicting Multicast DNS responses
are received, then the probing host MUST defer to the existing host,
and MUST choose new names for some or all of its resource records as
appropriate. In the case of a host probing using query type ANY as
recommended above, any answer containing a record with that name, of
any type, MUST be considered a conflicting response and handled
accordingly.
If fifteen failures occur within any ten-second period, then the host
MUST wait at least five seconds before each successive additional
probe attempt. This is to help ensure that in the event of software
bugs or other unanticipated problems, errant hosts do not flood the
network with a continuous stream of multicast traffic. For very
simple devices, a valid way to comply with this requirement is
to always wait five seconds after any failed probe attempt before
trying again.
If a Responder knows by other means, with absolute certainty, that
its unique resource record set name, rrtype and rrclass cannot
already be in use by any other Responder on the network, then it
MAY skip the probing step for that resource record set. For example,
when creating the reverse address mapping PTR records, the host can
reasonably assume that no other host will be trying to create those
same PTR records, since that would imply that the two hosts were
trying to use the same IP address, and if that were the case, the
two hosts would be suffering communication problems beyond the scope
of what Multicast DNS is designed to solve.
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9.2 Simultaneous Probe Tie-Breaking
The astute reader will observe that there is a race condition
inherent in the previous description. If two hosts are probing for
the same name simultaneously, neither will receive any response to
the probe, and the hosts could incorrectly conclude that they may
both proceed to use the name. To break this symmetry, each host
populates the Query packets's Authority Section with the record or
records with the rdata that it would be proposing to use, should its
probing be successful. The Authority Section is being used here in a
way analogous to the way it is used as the "Update Section" in a DNS
Update packet [RFC 2136].
When a host is probing for a group of related records with the same
name (e.g. the SRV and TXT record describing a DNS-SD service), only
a single question need be placed in the Question Section, since query
type ANY (255) is used, which will elicit answers for all records
with that name. However, for tie-breaking to work correctly in all
cases, the Authority Section must contain *all* the records and
proposed rdata being probed for uniqueness.
When a host that is probing for a record sees another host issue a
query for the same record, it consults the Authority Section of that
query. If it finds any resource record(s) there which answers the
query, then it compares the data of that (those) resource record(s)
with its own tentative data. We consider first the simple case of a
host probing for a single record, receiving a simultaneous probe from
another host also probing for a single record. The two records are
compared and the lexicographically later data wins. This means that
if the host finds that its own data is lexicographically later, it
simply ignores the other host's probe. If the host finds that its own
data is lexicographically earlier, then it treats this exactly as if
it had received a positive answer to its query, and concludes that it
may not use the desired name.
The determination of "lexicographically later" is performed by first
comparing the record class, then the record type, then raw comparison
of the binary content of the rdata without regard for meaning or
structure. If the record classes differ, then the numerically greater
class is considered "lexicographically later". Otherwise, if the
record types differ, then the numerically greater type is considered
"lexicographically later". If the rrtype and rrclass both match then
the rdata is compared.
In the case of resource records containing rdata that is subject to
name compression [RFC 1035], the names MUST be uncompressed before
comparison. (The details of how a particular name is compressed is an
artifact of how and where the record is written into the DNS message;
it is not an intrinsic property of the resource record itself.)
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The bytes of the raw uncompressed rdata are compared in turn,
interpreting the bytes as eight-bit UNSIGNED values, until a byte
is found whose value is greater than that of its counterpart (in
which case the rdata whose byte has the greater value is deemed
lexicographically later) or one of the resource records runs out
of rdata (in which case the resource record which still has
remaining data first is deemed lexicographically later).
The following is an example of a conflict:
cheshire.local. A 169.254.99.200
cheshire.local. A 169.254.200.50
In this case 169.254.200.50 is lexicographically later (the third
byte, with value 200, is greater than its counterpart with value 99),
so it is deemed the winner.
Note that it is vital that the bytes are interpreted as UNSIGNED
values in the range 0-255, or the wrong outcome may result. In
the example above, if the byte with value 200 had been incorrectly
interpreted as a signed eight-bit value then it would be interpreted
as value -56, and the wrong address record would be deemed the
winner.
9.2.1 Simultaneous Probe Tie-Breaking for Multiple Records
When a host is probing for a set of records with the same name, or a
packet is received containing multiple tie-breaker records answering
a given probe question in the Question Section, the host's records
and the tie-breaker records from the packet are each sorted into
order, and then compared pairwise, using the same comparison
technique described above, until a difference is found.
The records are sorted using the same lexicographical order as
described above, that is: if the record classes differ, the record
with the lower class number comes first. If the classes are the same
but the rrtypes differ, the record with the lower rrtype number comes
first. If the class and rrtype match, then the rdata is compared
bytewise until a difference is found. For example, in the common case
of advertising DNS-SD services with a TXT record and an SRV record,
the TXT record comes first (the rrtype for TXT is 16) and the SRV
record comes second (the rrtype for SRV is 33).
When comparing the records, if the first records match perfectly,
then the second records are compared, and so on. If either list of
records runs out of records before any difference is found, then the
list with records remaining is deemed to have won the tie-break. If
both lists run out of records at the same time without any difference
being found, then this indicates that two devices are advertising
identical sets of records, as is sometimes done for fault tolerance,
and there is in fact no conflict.
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9.3 Announcing
The second startup step is that the Multicast DNS Responder MUST send
a gratuitous Multicast DNS Response containing, in the Answer
Section, all of its resource records (both shared records, and unique
records that have completed the probing step). If there are too many
resource records to fit in a single packet, multiple packets should
be used.
In the case of shared records (e.g. the PTR records used by DNS
Service Discovery [DNS-SD]), the records are simply placed as-is
into the Answer Section of the DNS Response.
In the case of records that have been verified to be unique in the
previous step, they are placed into the Answer Section of the DNS
Response with the most significant bit of the rrclass set to one.
The most significant bit of the rrclass for a record in the Answer
Section of a response packet is the mDNS "cache flush" bit and is
discussed in more detail below in Section 11.3 "Announcements to
Flush Outdated Cache Entries".
The Multicast DNS Responder MUST send at least two gratuitous
responses, one second apart. A Responder MAY send up to eight
gratuitous Responses, provided that the interval between gratuitous
responses doubles with every response sent.
A Multicast DNS Responder MUST NOT send announcements in the absence
of information that its network connectivity may have changed in
some relevant way. In particular, a Multicast DNS Responder MUST NOT
send regular periodic announcements as a matter of course. It is not
uncommon for protocol designers to encounter some problem which they
decide to solve using regular periodic announcements, but this is
generally not a wise protocol design choice. In the small scale
periodic announcements may seem to remedy the short-term problem,
but they do not scale well if the protocol becomes successful.
If every host on the network implements the protocol -- if multiple
applications on every host on the network are implementing the
protocol -- then even a low periodic rate of just one announcement
per minute per application per host can add up to multiple packets
per second in total. While gigabit Ethernet may be able to carry
a million packets per second, other network technologies cannot.
For example, while IEEE 802.11g [IEEE W] wireless has a nominal data
rate of up to 54Mb/sec, multicasting just 100 packets per second can
consume the entire available bandwidth, leaving nothing for anything
else.
With the increasing popularity of hand-held devices, unnecessary
continuous packet transmission can have bad implications for battery
life. It's worth pointing out the precedent that TCP was also
designed with this "no regular periodic idle packets" philosophy.
Standard TCP sends packets only when it has data to send or
acknowledge. If neither client nor server sends any bytes, then the
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TCP code will send no packets, and a TCP connection can remain active
in this state indefinitely, with no packets being exchanged for
hours, days, weeks or months.
Whenever a Multicast DNS Responder receives any Multicast DNS
response (gratuitous or otherwise) containing a conflicting resource
record, the conflict MUST be resolved as described below in "Conflict
Resolution".
9.4 Updating
At any time, if the rdata of any of a host's Multicast DNS records
changes, the host MUST repeat the Announcing step described above to
update neighboring caches. For example, if any of a host's IP
addresses change, it MUST re-announce those address records.
In the case of shared records, a host MUST send a "goodbye"
announcement with TTL zero (see Section 11.2 "Goodbye Packets")
for the old rdata, to cause it to be deleted from peer caches,
before announcing the new rdata. In the case of unique records,
a host SHOULD omit the "goodbye" announcement, since the cache
flush bit on the newly announced records will cause old rdata
to be flushed from peer caches anyway.
A host may update the contents of any of its records at any time,
though a host SHOULD NOT update records more frequently than ten
times per minute. Frequent rapid updates impose a burden on the
network. If a host has information to disseminate which changes more
frequently than ten times per minute, then it may be more appropriate
to design a protocol for that specific purpose.
10. Conflict Resolution
A conflict occurs when a Multicast DNS Responder has a unique record
for which it is authoritative, and it receives a Multicast DNS
response packet containing a record with the same name, rrtype and
rrclass, but inconsistent rdata. What may be considered inconsistent
is context sensitive, except that resource records with identical
rdata are never considered inconsistent, even if they originate from
different hosts. This is to permit use of proxies and other
fault-tolerance mechanisms that may cause more than one Responder
to be capable of issuing identical answers on the network.
A common example of a resource record type that is intended to be
unique, not shared between hosts, is the address record that maps a
host's name to its IP address. Should a host witness another host
announce an address record with the same name but a different IP
address, then that is considered inconsistent, and that address
record is considered to be in conflict.
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Whenever a Multicast DNS Responder receives any Multicast DNS
response (gratuitous or otherwise) containing a conflicting resource
record in the Answer Section, the Multicast DNS Responder MUST
immediately reset its conflicted unique record to probing state, and
go through the startup steps described above in Section 9, "Probing
and Announcing on Startup". The protocol used in the Probing phase
will determine a winner and a loser, and the loser MUST cease using
the name, and reconfigure.
It is very important that any host receiving a resource record that
conflicts with one of its own MUST take action as described above.
In the case of two hosts using the same host name, where one has been
configured to require a unique host name and the other has not, the
one that has not been configured to require a unique host name will
not perceive any conflict, and will not take any action. By reverting
to Probing state, the host that desires a unique host name will go
through the necessary steps to ensure that a unique host name is
obtained.
The recommended course of action after probing and failing is as
follows:
o Programmatically change the resource record name in an attempt to
find a new name that is unique. This could be done by adding some
further identifying information (e.g. the model name of the
hardware) if it is not already present in the name, appending the
digit "2" to the name, or incrementing a number at the end of the
name if one is already present.
o Probe again, and repeat until a unique name is found.
o Record this newly chosen name in persistent storage so that the
device will use the same name the next time it is power-cycled.
o Display a message to the user or operator informing them of the
name change. For example:
The name "Bob's Music" is in use by another iTunes music
server on the network. Your music has been renamed to
"Bob's Music (MacBook)". If you want to change this name,
use [describe appropriate menu item or preference dialog].
o If after one minute of probing the Multicast DNS Responder has been
unable to find any unused name, it should display a message to the
user or operator informing them of this fact. This situation should
never occur in normal operation. The only situations that would
cause this to happen would be either a deliberate denial-of-service
attack, or some kind of very obscure hardware or software bug that
acts like a deliberate denial-of-service attack.
How the user or operator is informed depends on context. A desktop
computer with a screen might put up a dialog box. A headless server
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in the closet may write a message to a log file, or use whatever
mechanism (email, SNMP trap, etc.) it uses to inform the
administrator of error conditions. On the other hand a headless
server in the closet may not inform the user at all -- if the user
cares, they will notice the name has changed, and connect to the
server in the usual way (e.g. via Web Browser) to configure a new
name.
These considerations apply to address records (i.e. host names) and
to all resource records where uniqueness (or maintenance of some
other defined constraint) is desired.
11. Resource Record TTL Values and Cache Coherency
As a general rule, the recommended TTL value for Multicast DNS
resource records with a host name as the resource record's name
(e.g. A, AAAA, HINFO, etc.) or contained within the resource record's
rdata (e.g. SRV, reverse mapping PTR record, etc.) is 120 seconds.
The recommended TTL value for other Multicast DNS resource records
is 75 minutes.
A client with an active outstanding query will issue a query packet
when one or more of the resource record(s) in its cache is (are) 80%
of the way to expiry. If the TTL on those records is 75 minutes,
this ongoing cache maintenance process yields a steady-state query
rate of one query every 60 minutes.
Any distributed cache needs a cache coherency protocol. If Multicast
DNS resource records follow the recommendation and have a TTL of 75
minutes, that means that stale data could persist in the system for
a little over an hour. Making the default TTL significantly lower
would reduce the lifetime of stale data, but would produce too much
extra traffic on the network. Various techniques are available to
minimize the impact of such stale data.
11.1 Cooperating Multicast DNS Responders
If a Multicast DNS Responder ("A") observes some other Multicast DNS
Responder ("B") send a Multicast DNS Response packet containing a
resource record with the same name, rrtype and rrclass as one of A's
resource records, but different rdata, then:
o If A's resource record is intended to be a shared resource record,
then this is no conflict, and no action is required.
o If A's resource record is intended to be a member of a unique
resource record set owned solely by that Responder, then this
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is a conflict and MUST be handled as described in Section 10
"Conflict Resolution".
If a Multicast DNS Responder ("A") observes some other Multicast DNS
Responder ("B") send a Multicast DNS Response packet containing a
resource record with the same name, rrtype and rrclass as one of A's
resource records, and identical rdata, then:
o If the TTL of B's resource record given in the packet is at least
half the true TTL from A's point of view, then no action is
required.
o If the TTL of B's resource record given in the packet is less than
half the true TTL from A's point of view, then A MUST mark its
record to be announced via multicast. Clients receiving the record
from B would use the TTL given by B, and hence may delete the
record sooner than A expects. By sending its own multicast response
correcting the TTL, A ensures that the record will be retained for
the desired time.
These rules allow multiple Multicast DNS Responders to offer the same
data on the network (perhaps for fault tolerance reasons) without
conflicting with each other.
11.2 Goodbye Packets
In the case where a host knows that certain resource record data is
about to become invalid (for example when the host is undergoing a
clean shutdown) the host SHOULD send a gratuitous announcement mDNS
response packet, giving the same resource record name, rrtype,
rrclass and rdata, but an RR TTL of zero. This has the effect of
updating the TTL stored in neighboring hosts' cache entries to zero,
causing that cache entry to be promptly deleted.
Clients receiving a Multicast DNS Response with a TTL of zero SHOULD
NOT immediately delete the record from the cache, but instead record
a TTL of 1 and then delete the record one second later. In the case
of multiple Multicast DNS Responders on the network described in
Section 11.1 above, if one of the Responders shuts down and
incorrectly sends goodbye packets for its records, it gives the other
cooperating Responders one second to send out their own response to
"rescue" the records before they expire and are deleted.
11.3 Announcements to Flush Outdated Cache Entries
Whenever a host has a resource record with new data, or with what
might potentially be new data (e.g. after rebooting, waking from
sleep, connecting to a new network link, changing IP address, etc.),
the host needs to inform peers of that new data. In cases where the
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host has not been continuously connected and participating on the
network link, it MUST first Probe to re-verify uniqueness of its
unique records, as described above in Section 9.1 "Probing".
Having completed the Probing step if necessary, the host MUST then
send a series of gratuitous announcements to update cache entries in
its neighbor hosts. In these gratuitous announcements, if the record
is one that has been verified unique, the host sets the most
significant bit of the rrclass field of the resource record. This
bit, the "cache flush" bit, tells neighboring hosts that this is not
a shared record type. Instead of merging this new record additively
into the cache in addition to any previous records with the same
name, rrtype and rrclass, all old records with that name, type and
class that were received more than one second ago are declared
invalid, and marked to expire from the cache in one second.
The semantics of the cache flush bit are as follows: Normally when a
resource record appears in the Answer Section of the DNS Response, it
means, "This is an assertion that this information is true." When a
resource record appears in the Answer Section of the DNS Response
with the "cache flush" bit set, it means, "This is an assertion that
this information is the truth and the whole truth, and anything you
may have heard more than a second ago regarding records of this
name/rrtype/rrclass is no longer valid".
To accommodate the case where the set of records from one host
constituting a single unique RRSet is too large to fit in a single
packet, only cache records that are more than one second old are
flushed. This allows the announcing host to generate a quick burst of
packets back-to-back on the wire containing all the members
of the RRSet. When receiving records with the "cache flush" bit set,
all records older than one second are marked to be deleted one second
in the future. One second after the end of the little packet burst,
any records not represented within that packet burst will then be
expired from all peer caches.
Any time a host sends a response packet containing some members of a
unique RRSet, it SHOULD send the entire RRSet, preferably in a single
packet, or if the entire RRSet will not fit in a single packet, in a
quick burst of packets sent as close together as possible. The host
SHOULD set the cache flush bit on all members of the unique RRSet.
In the event that for some reason the host chooses not to send the
entire unique RRSet in a single packet or a rapid packet burst,
it MUST NOT set the cache flush bit on any of those records.
The reason for waiting one second before deleting stale records from
the cache is to accommodate bridged networks. For example, a host's
address record announcement on a wireless interface may be bridged
onto a wired Ethernet, and cause that same host's Ethernet address
records to be flushed from peer caches. The one-second delay gives
the host the chance to see its own announcement arrive on the wired
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Ethernet, and immediately re-announce its Ethernet interface's
address records so that both sets remain valid and live in peer
caches.
These rules, about when to set the cache flush bit and sending the
entire rrset, apply regardless of *why* the response packet is being
generated. They apply to startup announcements as described in
Section 9.3 "Announcing", and to responses generated as a result
of receiving query packets.
The "cache flush" bit is only set in records in the Answer Section of
Multicast DNS responses sent to UDP port 5353. The "cache flush" bit
MUST NOT be set in any resource records in a response packet sent in
legacy unicast responses to UDP ports other than 5353.
The "cache flush" bit MUST NOT be set in any resource records in the
known-answer list of any query packet.
The "cache flush" bit MUST NOT ever be set in any shared resource
record. To do so would cause all the other shared versions of this
resource record with different rdata from different Responders to be
immediately deleted from all the caches on the network.
The "cache flush" bit does *not* apply to questions listed in the
Question Section of a Multicast DNS packet. The top bit of the
rrclass field in questions is used for an entirely different purpose
(see Section 6.5, "Questions Requesting Unicast Responses").
Note that the "cache flush" bit is NOT part of the resource record
class. The "cache flush" bit is the most significant bit of the
second 16-bit word of a resource record in the Answer Section of
an mDNS packet (the field conventionally referred to as the rrclass
field), and the actual resource record class is the least-significant
fifteen bits of this field. There is no mDNS resource record class
0x8001. The value 0x8001 in the rrclass field of a resource record in
an mDNS response packet indicates a resource record with class 1,
with the "cache flush" bit set. When receiving a resource record with
the "cache flush" bit set, implementations should take care to mask
off that bit before storing the resource record in memory.
The re-use of the top bit of the rrclass field only applies to
conventional Resource Record types that are subject to caching, not
to pseudo-RRs like OPT [RFC 2671], TSIG [RFC 2845], TKEY [RFC 2930],
SIG0 [RFC 2931], etc., that pertain only to a particular transport
level message and not to any actual DNS data. Since pseudo-RRs should
never go into the mDNS cache, the concept of a "cache flush" bit for
these types is not applicable. In particular the rrclass field of
an OPT records encodes the sender's UDP payload size, and should
be interpreted as a 16-bit length value in the range 0-65535, not
a one-bit flag and a 15-bit length.
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11.4 Cache Flush on Topology change
If the hardware on a given host is able to indicate physical changes
of connectivity, then when the hardware indicates such a change, the
host should take this information into account in its mDNS cache
management strategy. For example, a host may choose to immediately
flush all cache records received on a particular interface when that
cable is disconnected. Alternatively, a host may choose to adjust the
remaining TTL on all those records to a few seconds so that if the
cable is not reconnected quickly, those records will expire from the
cache.
Likewise, when a host reboots, or wakes from sleep, or undergoes some
other similar discontinuous state change, the cache management
strategy should take that information into account.
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11.5 Cache Flush on Failure Indication
Sometimes a cache record can be determined to be stale when a client
attempts to use the rdata it contains, and finds that rdata to be
incorrect.
For example, the rdata in an address record can be determined to be
incorrect if attempts to contact that host fail, either because
ARP/ND requests for that address go unanswered (for an address on a
local subnet) or because a router returns an ICMP "Host Unreachable"
error (for an address on a remote subnet).
The rdata in an SRV record can be determined to be incorrect if
attempts to communicate with the indicated service at the host and
port number indicated are not successful.
The rdata in a DNS-SD PTR record can be determined to be incorrect if
attempts to look up the SRV record it references are not successful.
In any such case, the software implementing the mDNS resource record
cache should provide a mechanism so that clients detecting stale
rdata can inform the cache.
When the cache receives this hint that it should reconfirm some
record, it MUST issue two or more queries for the resource record in
question. If no response is received in a reasonable amount of time,
then, even though its TTL may indicate that it is not yet due to
expire, that record SHOULD be promptly flushed from the cache.
The end result of this is that if a printer suffers a sudden power
failure or other abrupt disconnection from the network, its name
may continue to appear in DNS-SD browser lists displayed on users'
screens. Eventually that entry will expire from the cache naturally,
but if a user tries to access the printer before that happens, the
failure to successfully contact the printer will trigger the more
hasty demise of its cache entries. This is a sensible trade-off
between good user-experience and good network efficiency. If we were
to insist that printers should disappear from the printer list within
30 seconds of becoming unavailable, for all failure modes, the only
way to achieve this would be for the client to poll the printer at
least every 30 seconds, or for the printer to announce its presence
at least every 30 seconds, both of which would be an unreasonable
burden on most networks.
11.6 Passive Observation of Failures
A host observes the multicast queries issued by the other hosts on
the network. One of the major benefits of also sending responses
using multicast is that it allows all hosts to see the responses (or
lack thereof) to those queries.
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If a host sees queries, for which a record in its cache would be
expected to be given as an answer in a multicast response, but no
such answer is seen, then the host may take this as an indication
that the record may no longer be valid.
After seeing two or more of these queries, and seeing no multicast
response containing the expected answer within a reasonable amount of
time, then even though its TTL may indicate that it is not yet due to
expire, that record MAY be flushed from the cache. The host SHOULD
NOT perform its own queries to re-confirm that the record is truly
gone. If every host on a large network were to do this, it would
cause a lot of unnecessary multicast traffic. If host A sends
multicast queries that remain unanswered, then there is no reason
to suppose that host B or any other host is likely to be any more
successful.
The previous section, "Cache Flush on Failure Indication", describes
a situation where a user trying to print discovers that the printer
is no longer available. By implementing the passive observation
described here, when one user fails to contact the printer, all
hosts on the network observe that failure and update their caches
accordingly.
12. Special Characteristics of Multicast DNS Domains
Unlike conventional DNS names, names that end in ".local." or
"254.169.in-addr.arpa." have only local significance. The same is
true of names within the IPv6 Link-Local reverse mapping domains.
Conventional Unicast DNS seeks to provide a single unified namespace,
where a given DNS query yields the same answer no matter where on the
planet it is performed or to which recursive DNS server the query is
sent. In contrast, each IP link has its own private ".local.",
"254.169.in-addr.arpa." and IPv6 Link-Local reverse mapping
namespaces, and the answer to any query for a name within those
domains depends on where that query is asked. (This characteristic is
not unique to Multicast DNS. Although the original concept of DNS was
a single global namespace, in recent years split views, firewalls,
intranets, and the like have increasingly meant that the answer to a
given DNS query has become dependent on the location of the querier.)
The IPv4 name server for a Multicast DNS Domain is 224.0.0.251. The
IPv6 name server for a Multicast DNS Domain is FF02::FB. These are
multicast addresses; therefore they identify not a single host but a
collection of hosts, working in cooperation to maintain some
reasonable facsimile of a competently managed DNS zone. Conceptually
a Multicast DNS Domain is a single DNS zone, however its server is
implemented as a distributed process running on a cluster of loosely
cooperating CPUs rather than as a single process running on a single
CPU.
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Multicast DNS Domains are not delegated from their parent domain via
use of NS records, and there is also no concept of delegation of
subdomains within a Multicast DNS Domain. Just because a particular
host on the network may answer queries for a particular record type
with the name "example.local." does not imply anything about whether
that host will answer for the name "child.example.local.", or indeed
for other record types with the name "example.local."
There are no NS records anywhere in Multicast DNS Domains. Instead,
the Multicast DNS Domains are reserved by IANA and there is
effectively an implicit delegation of all Multicast DNS Domains to
the IP addresses 224.0.0.251 and FF02::FB, by virtue of client
software implementing the protocol rules specified in this document.
Multicast DNS Zones have no SOA record. A conventional DNS zone's
SOA record contains information such as the email address of the zone
administrator and the monotonically increasing serial number of the
last zone modification. There is no single human administrator for
any given Multicast DNS Zone, so there is no email address. Because
the hosts managing any given Multicast DNS Zone are only loosely
coordinated, there is no readily available monotonically increasing
serial number to determine whether or not the zone contents have
changed. A host holding part of the shared zone could crash or be
disconnected from the network at any time without informing the other
hosts. There is no reliable way to provide a zone serial number that
would, whenever such a crash or disconnection occurred, immediately
change to indicate that the contents of the shared zone had changed.
Zone transfers are not possible for any Multicast DNS Zone.
13. Multicast DNS for Service Discovery
This document does not describe using Multicast DNS for network
browsing or service discovery. However, the mechanisms this document
describes are compatible with (and support) the browsing and service
discovery mechanisms specified in "DNS-Based Service Discovery"
[DNS-SD].
14. Enabling and Disabling Multicast DNS
The option to fail-over to Multicast DNS for names not ending
in ".local." SHOULD be a user-configured option, and SHOULD
be disabled by default because of the possible security issues
related to unintended local resolution of apparently global names.
The option to lookup unqualified (relative) names by appending
".local." (or not) is controlled by whether ".local." appears
(or not) in the client's DNS search list.
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No special control is needed for enabling and disabling Multicast DNS
for names explicitly ending with ".local." as entered by the user.
The user doesn't need a way to disable Multicast DNS for names ending
with ".local.", because if the user doesn't want to use Multicast
DNS, they can achieve this by simply not using those names. If a user
*does* enter a name ending in ".local.", then we can safely assume
the user's intention was probably that it should work. Having user
configuration options that can be (intentionally or unintentionally)
set so that local names don't work is just one more way of
frustrating the user's ability to perform the tasks they want,
perpetuating the view that, "IP networking is too complicated to
configure and too hard to use." This perception prolonged the
continued use of protocols like AppleTalk and NetBIOS long after they
should have been retired, and continues to encourage the creation of
new one-off hardware-specific protocols. If we want to stop this
pointless duplication of effort, we need to provide IP functionality
that users can rely on to "always work, like AppleTalk." A little
Multicast DNS traffic may be a burden on the network, but it is an
insignificant burden compared to the continued use of AppleTalk and
the creation of yet more protocols like it.
15. Considerations for Multiple Interfaces
A host SHOULD defend its host name (FQDN) on all active interfaces on
which it is answering Multicast DNS queries.
In the event of a name conflict on *any* interface, a host should
configure a new host name, if it wishes to maintain uniqueness of its
host name.
A host may choose to use the same name for all of its address records
on all interfaces, or it may choose to manage its Multicast DNS host
name(s) independently on each interface, potentially answering to
different names on different interfaces.
When answering a Multicast DNS query, a multi-homed host with a
link-local address (or addresses) SHOULD take care to ensure that
any address going out in a Multicast DNS response is valid for use
on the interface on which the response is going out.
Just as the same link-local IP address may validly be in use
simultaneously on different links by different hosts, the same
link-local host name may validly be in use simultaneously on
different links, and this is not an error. A multi-homed host with
connections to two different links may be able to communicate with
two different hosts that are validly using the same name. While this
kind of name duplication should be rare, it means that a host that
wants to fully support this case needs network programming APIs that
allow applications to specify on what interface to perform a
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link-local Multicast DNS query, and to discover on what interface a
Multicast DNS response was received.
There is one other special precaution that multi-homed hosts need to
take. It's common with today's laptop computers to have an Ethernet
connection and an 802.11 [IEEE W] wireless connection active at the
same time. What the software on the laptop computer can't easily tell
is whether the wireless connection is in fact bridged onto the same
network segment as its Ethernet connection. If the two networks are
bridged together, then packets the host sends on one interface will
arrive on the other interface a few milliseconds later, and care must
be taken to ensure that this bridging does not cause problems:
When the host announces its host name (i.e. its address records) on
its wireless interface, those announcement records are sent with the
cache-flush bit set, so when they arrive on the Ethernet segment,
they will cause all the peers on the Ethernet to flush the host's
Ethernet address records from their caches. The mDNS protocol has a
safeguard to protect against this situation: when records are
received with the cache-flush bit set, other records are not deleted
from peer caches immediately, but are marked for deletion in one
second. When the host sees its own wireless address records arrive on
its Ethernet interface, with the cache-flush bit set, this one-second
grace period gives the host time to respond and re-announce its
Ethernet address records, to reinstate those records in peer caches
before they are deleted.
As described, this solves one problem, but creates another, because
when those Ethernet announcement records arrive back on the wireless
interface, the host would again respond defensively to reinstate its
wireless records, and this process would continue forever,
continuously flooding the network with traffic. The mDNS protocol has
a second safeguard, to solve this problem: the cache-flush bit does
not apply to records received very recently, within the last second.
This means that when the host sees its own Ethernet address records
arrive on its wireless interface, with the cache-flush bit set, it
knows there's no need to re-announce its wireless address records
again because it already sent them less than a second ago, and this
makes them immune from deletion from peer caches.
16. Considerations for Multiple Responders on the Same Machine
It is possible to have more than one Multicast DNS Responder and/or
Querier implementation coexist on the same machine, but there are
some known issues.
16.1 Receiving Unicast Responses
In most operating systems, incoming multicast packets can be
delivered to *all* open sockets bound to the right port number,
provided that the clients take the appropriate steps to allow this.
For this reason, all Multicast DNS implementations SHOULD use the
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SO_REUSEPORT and/or SO_REUSEADDR options (or equivalent as
appropriate for the operating system in question) so they will all be
able to bind to UDP port 5353 and receive incoming multicast packets
addressed to that port. However, incoming unicast UDP packets are
typically delivered only to the first socket to bind to that port.
This means that "QU" responses and other packets sent via unicast
will be received only by the first Multicast DNS Responder and/or
Querier on a system. This limitation can be partially mitigated if
Multicast DNS implementations detect when they are not the first
to bind to port 5353, and in that case they do not request "QU"
responses. One way to detect if there is another Multicast DNS
implementation already running is to attempt binding to port 5353
without using SO_REUSEPORT and/or SO_REUSEADDR, and if that fails
it indicates that some other socket is already bound to this port.
16.2 Multi-Packet Known-Answer lists
When a Multicast DNS Querier issues a query with too many known
answers to fit into a single packet, it divides the known answer list
into two or more packets. Multicast DNS Responders associate the
initial truncated query with its continuation packets by examining
the source IP address in each packet. Since two independent Multicast
DNS Queriers running on the same machine will be sending packets with
the same source IP address, from an outside perspective they appear
to be a single entity. If both Queriers happened to send the same
multi-packet query at the same time, with different known answer
lists, then they could each end up suppressing answers that the other
needs.
16.3 Efficiency
If different clients on a machine were to each have their own
separate independent Multicast DNS implementation, they would lose
certain efficiency benefits. Apart from the unnecessary code
duplication, memory usage, and CPU load, the clients wouldn't get the
benefit of a shared system-wide cache, and they would not be able to
aggregate separate queries into single packets to reduce network
traffic.
16.4 Recommendation
Because of these issues, this document encourages implementers to
design systems with a single Multicast DNS implementation that
provides Multicast DNS services shared by all clients on that
machine, much as most operating systems today have a single TCP
implementation, which is shared between all clients on that machine.
Due to engineering constraints, there may be situations where
embedding a Multicast DNS implementation in the client is the most
expedient solution, and while this will usually work in practice,
implementers should be aware of the issues outlined in this section.
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17. Multicast DNS and Power Management
Many modern network devices have the ability to go into a low-power
mode where only a small part of the Ethernet hardware remains
powered, and the device can be woken up by sending a specially
formatted Ethernet frame which the device's power-management hardware
recognizes. Ethernet hardware with this "Wake on LAN" or "Magic
Packet" capability has been available since 1997 [WoL], and recently
vendors have also started to offer 802.11 wireless devices [IEEE W]
with similar "Magic Packet" network wakeup capabilities.
Sadly, despite having been available for nearly two decades, the vast
majority of computer users -- especially home users -- have never
made use of this Wake on LAN capability, which would allow them to
save power by putting their computers to sleep, and then waking them
over the network only when needed. Furthermore, over the course of
those two decades, computer vendors have been busy creating network-
oriented products and services that encourage users to not let their
computers sleep:
o A desktop computer can share its locally-attached USB printer,
allowing users to sit on the sofa with their WiFi-enabled laptops
and print documents to that printer -- but only if the desktop
computer is not asleep.
o Set-top boxes (e.g. Apple's "Apple TV") connected to a television
can play music, photographic slide shows, and movies stored on the
user's desktop computer (e.g. an iMac running iTunes) -- but only
if that desktop computer is not asleep.
o Services like Apple's "Back to My Mac" allow users to access data
on their home computers from remote locations, using screen
sharing or file sharing -- but only if their computer at home
is not asleep.
By combining "Wake on LAN" with Multicast DNS, Wake on LAN can be
made automatic and effortless, so that all users can get the
power-savings benefit it offers, instead of being limited to use by
only a small minority of computer experts, who know how to write
their own scripts or use specialized tools to generate the "Magic
Packet" manually.
To make "Wake on LAN" automatic and effortless for everyone, we have
created a network power management service called Sleep Proxy
Service. A device that wishes to enter low-power mode first uses
Multicast DNS-SD to discover if Sleep Proxy Service is available on
the local network. In some networks there may be more than one piece
of hardware implementing Sleep Proxy Service, for fault-tolerance
reasons.
If the device finds the network has Sleep Proxy Service, then the
device transfers its Multicast DNS records to the Sleep Proxy using a
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DNS Update packet [RFC 2136]. In addition, the device includes in
that DNS Update packet an EDNS0 OPT record [RFC 2671] containing an
'owner' option [OWNER], which tells the Sleep Proxy how to build the
appropriate Magic Packet to wake up the device. This small collection
of Multicast DNS records (typically small enough to fit in a single
UDP packet) provides an efficient compact representation of the
device's role on the network -- its hostname, its IPv4 and IPv6
addresses, what services it offers, their names, what TCP and UDP
ports it is listening on, and so on.
When a Sleep Proxy sees an mDNS query for one of the sleeping
device's records (e.g. a DNS-SD PTR record), it answers on behalf of
the sleeping device, without waking it up.
When a Sleep Proxy sees an IPv4 ARP or IPv6 ND Request for one of the
sleeping device's addresses, it answers on behalf of the sleeping
device, without waking it up, giving its own MAC address as the
current (temporary) owner of that address.
By claiming (temporary) ownership of the sleeping device's
address(es), when local peers send packets addressed to that sleeping
device (including routers on the local link sending packets
addressed to the sleeping device on behalf of remote hosts
communicating with it), those packets will go to the Sleep Proxy.
The Sleep Proxy software can receive those packets (either within
the kernel, or using a user-level packet capture facility such as
Berkeley Packet Filter [BPF]) and inspects them to see if they
warrant waking the sleeping device. For example, a Sleep Proxy may
choose to wake a sleeping device when it receives a TCP SYN packet
requesting a new connection to one of the TCP ports upon which the
sleeping device has a listening socket. A Sleep Proxy may choose NOT
to wake a sleeping device when it receives a packet addressed to a
port which the sleeping device has not indicated that it is listening
on, since the device would be likely to simply discard such a packet
anyway.
When the Sleep Proxy determines that it is appropriate to wake the
sleeping device, it proceeds to send a series of "magic packets" to
wake the device up. When the Sleep Proxy observes Multicast DNS
packets from the device, containing the device's OWNER option, with
the OWNER sequence number incremented to signify a new period of
wakefulness, the Sleep Proxy can cease sending magic packets for that
device and discard any Multicast DNS records and other state it had
pertaining to its role as proxy for that sleeping device.
The connecting client does not need to be aware of how Sleep Proxy
Service works. It merely attempts a connection to the sleeping
device, and the sleeping device magically wakes.
This description in this section is intended to provide an overview
of how the DNS-SD/mDNS Sleep Proxy Service works, but this
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description alone is probably not sufficient for someone to create an
independent implementation. The full specification of the Sleep Proxy
Service is to be described in a future document. In the meantime,
more information can be found by consulting the mDNSResponder source
code at www.macosforge.org, which includes a full implementation of
the DNS-SD/mDNS Sleep Proxy Service, available under the Apache 2.0
Open Source license.
18. Multicast DNS Character Set
Historically, unicast DNS has been plagued by the lack of any support
for non-US characters. Indeed, conventional DNS is usually limited to
just letters, digits and hyphens, not even allowing spaces or other
punctuation. Attempts to remedy this for unicast DNS have been badly
constrained by the perceived need to accommodate old buggy legacy DNS
implementations. In reality, the DNS specification actually imposes
no limits on what characters may be used in names, and good DNS
implementations handle any arbitrary eight-bit data without trouble.
"Clarifications to the DNS Specification" [RFC 2181] directly
discusses the subject of allowable character set in Section 11 ("Name
syntax"), and explicitly states that DNS names may contain arbitrary
eight-bit data. However, the old rules for ARPANET host names back in
the 1980s required host names to be just letters, digits, and hyphens
[RFC 1034], and since the predominant use of DNS is to store host
address records, many have assumed that the DNS protocol itself
suffers from the same limitation. It might be accurate to say that
there could be hypothetical bad implementations that do not handle
eight-bit data correctly, but it would not be accurate to say that
the protocol doesn't allow names containing eight-bit data.
Multicast DNS is a new protocol and doesn't (yet) have old buggy
legacy implementations to constrain the design choices. Accordingly,
it adopts the simple obvious elegant solution: all names in Multicast
DNS are encoded using precomposed UTF-8 [RFC 3629]. The characters
SHOULD conform to Unicode Normalization Form C (NFC) [UAX15]: Use
precomposed characters instead of combining sequences where possible,
e.g. use U+00C4 ("Latin capital letter A with diaeresis") instead of
U+0041 U+0308 ("Latin capital letter A", "combining diaeresis").
Some users of 16-bit Unicode have taken to stuffing a "zero-width
non-breaking space" character (U+FEFF) at the start of each UTF-16
file, as a hint to identify whether the data is big-endian or
little-endian, and calling it a "Byte Order Mark" (BOM). Since there
is only one possible byte order for UTF-8 data, a BOM is neither
necessary nor permitted. Multicast DNS names MUST NOT contain a "Byte
Order Mark". Any occurrence of the Unicode character U+FEFF at the
start or anywhere else in a Multicast DNS name MUST be interpreted as
being an actual intended part of the name, representing (just as for
any other legal unicode value) an actual literal instance of that
character (in this case a zero-width non-breaking space character).
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For names that are restricted to letters, digits and hyphens, the
UTF-8 encoding is identical to the US-ASCII encoding, so this is
entirely compatible with existing host names. For characters outside
the US-ASCII range, UTF-8 encoding is used.
Multicast DNS implementations MUST NOT use any other encodings apart
from precomposed UTF-8 (US-ASCII being considered a compatible subset
of UTF-8).
The backwards-compatibility issue mentioned above bears repeating:
After many years of debate, as a result of the perceived need to
accommodate certain DNS implementations that apparently couldn't
handle any character that's not a letter, digit or hyphen (and
apparently never will be updated to remedy this limitation) the
unicast DNS community settled on an extremely baroque encoding called
"Punycode" [RFC 3492]. Punycode is a remarkably ingenious encoding
solution, but it is complicated, hard to understand, and hard to
implement, using sophisticated techniques including insertion unsort
coding, generalized variable-length integers, and bias adaptation.
The resulting encoding is remarkably compact given the constraints,
but it's still not as good as simple straightforward UTF-8, and it's
hard even to predict whether a given input string will encode to a
Punycode string that fits within DNS's 63-byte limit, except by
simply trying the encoding and seeing whether it fits. Indeed, the
encoded size depends not only on the input characters, but on the
order they appear, so the same set of characters may or may not
encode to a legal Punycode string that fits within DNS's 63-byte
limit, depending on the order the characters appear. This is
extremely hard to present in a user interface that explains to users
why one name is allowed, but another name containing the exact same
characters is not. Neither Punycode nor any other of the "Ascii
Compatible Encodings" proposed for Unicast DNS may be used in
Multicast DNS packets. Any text being represented internally in some
other representation MUST be converted to canonical precomposed UTF-8
before being placed in any Multicast DNS packet.
The simple rules for case-insensitivity in Unicast DNS also apply in
Multicast DNS; that is to say, in name comparisons, the lower-case
letters "a" to "z" (0x61 to 0x7A) match their upper-case equivalents
"A" to "Z" (0x41 to 0x5A). Hence, if a client issues a query for an
address record with the name "cheshire.local.", then a Responder
having an address record with the name "Cheshire.local." should
issue a response. No other automatic equivalences should be assumed.
In particular all UTF-8 multi-byte characters (codes 0x80 and higher)
are compared by simple binary comparison of the raw byte values.
Accented characters are *not* defined to be automatically equivalent
to their unaccented counterparts. Where automatic equivalences are
desired, this may be achieved through the use of programmatically-
generated CNAME records. For example, if a Responder has an address
record for an accented name Y, and a client issues a query for a name
X, where X is the same as Y with all the accents removed, then the
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Responder may issue a response containing two resource records:
A CNAME record "X CNAME Y", asserting that the requested name X
(unaccented) is an alias for the true (accented) name Y, followed
by the address record for Y.
19. Multicast DNS Message Size
RFC 1035 restricts DNS Messages carried by UDP to no more than 512
bytes (not counting the IP or UDP headers) [RFC 1035]. For UDP
packets carried over the wide-area Internet in 1987, this was
appropriate. For link-local multicast packets on today's networks,
there is no reason to retain this restriction. Given that the packets
are by definition link-local, there are no Path MTU issues to
consider.
Multicast DNS Messages carried by UDP may be up to the IP MTU of the
physical interface, less the space required for the IP header (20
bytes for IPv4; 40 bytes for IPv6) and the UDP header (8 bytes).
In the case of a single mDNS Resource Record which is too large to
fit in a single MTU-sized multicast response packet, a Multicast DNS
Responder SHOULD send the Resource Record alone, in a single IP
datagram, sent using multiple IP fragments. Resource Records this
large SHOULD be avoided, except in the very rare cases where they
really are the appropriate solution to the problem at hand.
Implementers should be aware that many simple devices do not
re-assemble fragmented IP datagrams, so large Resource Records
SHOULD NOT be used except in specialized cases where the implementer
knows that all receivers implement reassembly.
A Multicast DNS packet larger than the interface MTU, which is sent
using fragments, MUST NOT contain more than one Resource Record.
Even when fragmentation is used, a Multicast DNS packet, including IP
and UDP headers, MUST NOT exceed 9000 bytes. 9000 bytes is the
maximum payload size of an Ethernet "Jumbo" packet, which makes it a
convenient upper limit to specify for the maximum Multicast DNS
packet size. (In practice Ethernet "Jumbo" packets are not widely
used, so it is advantageous to keep packets under 1500 bytes whenever
possible.)
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20. Multicast DNS Message Format
This section describes specific rules pertaining to the allowable
values for the header fields of a Multicast DNS message, and other
message format considerations.
20.1 ID (Query Identifier)
Multicast DNS clients SHOULD listen for gratuitous responses
issued by hosts booting up (or waking up from sleep or otherwise
joining the network). Since these gratuitous responses may contain a
useful answer to a question for which the client is currently
awaiting an answer, Multicast DNS clients SHOULD examine all received
Multicast DNS response messages for useful answers, without regard to
the contents of the ID field or the Question Section. In Multicast
DNS, knowing which particular query message (if any) is responsible
for eliciting a particular response message is less interesting than
knowing whether the response message contains useful information.
Multicast DNS clients MAY cache any or all Multicast DNS response
messages they receive, for possible future use, provided of course
that normal TTL aging is performed on these cached resource records.
In multicast query messages, the Query ID SHOULD be set to zero on
transmission.
In multicast responses, including gratuitous multicast responses, the
Query ID MUST be set to zero on transmission, and MUST be ignored on
reception.
In unicast response messages generated specifically in response to a
particular (unicast or multicast) query, the Query ID MUST match the
ID from the query message.
20.2 QR (Query/Response) Bit
In query messages, MUST be zero.
In response messages, MUST be one.
20.3 OPCODE
In both multicast query and multicast response messages, MUST be zero
(only standard queries are currently supported over multicast, unless
other queries are allowed by some future extension to the Multicast
DNS specification).
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20.4 AA (Authoritative Answer) Bit
In query messages, the Authoritative Answer bit MUST be zero on
transmission, and MUST be ignored on reception.
In response messages for Multicast Domains, the Authoritative Answer
bit MUST be set to one (not setting this bit would imply there's some
other place where "better" information may be found) and MUST be
ignored on reception.
20.5 TC (Truncated) Bit
In query messages, if the TC bit is set, it means that additional
Known Answer records may be following shortly. A Responder SHOULD
record this fact, and wait for those additional Known Answer records,
before deciding whether to respond. If the TC bit is clear, it means
that the querying host has no additional Known Answers.
In multicast response messages, the TC bit MUST be zero on
transmission, and MUST be ignored on reception.
In legacy unicast response messages, the TC bit has the same meaning
as in conventional unicast DNS: it means that the response was too
large to fit in a single packet, so the client SHOULD re-issue its
query using TCP in order to receive the larger response.
20.6 RD (Recursion Desired) Bit
In both multicast query and multicast response messages, the
Recursion Desired bit SHOULD be zero on transmission, and MUST be
ignored on reception.
20.7 RA (Recursion Available) Bit
In both multicast query and multicast response messages, the
Recursion Available bit MUST be zero on transmission, and MUST be
ignored on reception.
20.8 Z (Zero) Bit
In both query and response messages, the Zero bit MUST be zero on
transmission, and MUST be ignored on reception.
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20.9 AD (Authentic Data) Bit [RFC 2535]
In both multicast query and multicast response messages the Authentic
Data bit MUST be zero on transmission, and MUST be ignored on
reception.
20.10 CD (Checking Disabled) Bit [RFC 2535]
In both multicast query and multicast response messages, the Checking
Disabled bit MUST be zero on transmission, and MUST be ignored on
reception.
20.11 RCODE (Response Code)
In both multicast query and multicast response messages, the Response
Code MUST be zero on transmission. Multicast DNS messages received
with non-zero Response Codes MUST be silently ignored.
20.12 Repurposing of top bit of qclass in Question Section
In the Question Section of a Multicast DNS Query, the top bit of the
qclass field is used to indicate that unicast responses are preferred
for this particular question.
20.13 Repurposing of top bit of rrclass in Answer Section
In the Answer Section of a Multicast DNS Response, the top bit of the
rrclass field is used to indicate that the record is a member of a
unique RRSet, and the entire RRSet has been sent together (in the
same packet, or in consecutive packets if there are too many records
to fit in a single packet).
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20.14 Name Compression
When generating Multicast DNS packets, implementations SHOULD use
name compression wherever possible to compress the names of resource
records, by replacing some or all of the resource record name with a
compact two-byte reference to an appearance of that data somewhere
earlier in the packet [RFC 1035].
This applies not only to Multicast DNS Responses, but also to
Queries. When a Query contains more than one question, successive
questions often contain similar names, and consequently name
compression SHOULD be used, to save bytes. In addition, Queries may
also contain Known Answers in the Answer Section, or probe
tie-breaking data in the Authority Section, and these names SHOULD
similarly be compressed for network efficiency.
In addition to compressing the *names* of resource records, names
that appear within the *rdata* of the following rrtypes SHOULD also
be compressed in all Multicast DNS packets:
NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC
Implementations receiving Multicast DNS packets MUST correctly decode
compressed names appearing in the Question Section, and compressed
names of resource records appearing in other sections.
In addition, implementations MUST correctly decode compressed
names appearing within the *rdata* of the rrtypes listed above.
Where possible, implementations SHOULD also correctly decode
compressed names appearing within the *rdata* of other rrtypes known
to the implementers at the time of implementation, because such
forward-thinking planning helps facilitate the deployment of future
implementations that may have reason to compress those rrtypes.
One specific difference between Unicast DNS and Multicast DNS is that
Unicast DNS does not allow name compression for the target host in an
SRV record, because Unicast DNS implementations before the first SRV
specification in 1996 [RFC 2052] may not decode these compressed
records properly. Since all Multicast DNS implementations were
created after 1996, all Multicast DNS implementations are REQUIRED to
decode compressed SRV records correctly.
In legacy unicast responses generated to answer legacy queries, name
compression MUST NOT be performed on SRV records.
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21. Choice of UDP Port Number
Arguments were made for and against using Multicast on UDP port 53.
The final decision was to use UDP port 5353. Some of the arguments
for and against are given below.
21.1 Arguments for using UDP port 53:
* This is "just DNS", so it should be the same port.
* There is less work to be done updating old clients to do simple
mDNS queries. Only the destination address need be changed.
In some cases, this can be achieved without any code changes,
just by adding the address 224.0.0.251 to a configuration file.
21.2 Arguments for using a different port (UDP port 5353):
* This is not "just DNS". This is a DNS-like protocol, but different.
* Changing client code to use a different port number is not hard.
* Using the same port number makes it hard to run an mDNS Responder
and a conventional unicast DNS server on the same machine. If a
conventional unicast DNS server wishes to implement mDNS as well,
it can still do that, by opening two sockets. Having two different
port numbers allows this flexibility.
* Some VPN software hijacks all outgoing traffic to port 53 and
redirects it to a special DNS server set up to serve those VPN
clients while they are connected to the corporate network. It is
questionable whether this is the right thing to do, but it is
common, and redirecting link-local multicast DNS packets to a
remote server rarely produces any useful results. It does mean,
for example, that the user becomes unable to access their local
network printer sitting on their desk right next to their computer.
Using a different UDP port helps avoid this particular problem.
* On many operating systems, unprivileged clients may not send or
receive packets on low-numbered ports. This means that any client
sending or receiving mDNS packets on port 53 would have to run
as "root", which is an undesirable security risk. Using a higher-
numbered UDP port avoids this restriction.
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22. Summary of Differences Between Multicast DNS and Unicast DNS
The value of Multicast DNS is that it shares, as much as possible,
the familiar APIs, naming syntax, resource record types, etc., of
Unicast DNS. There are of course necessary differences by virtue of
it using multicast, and by virtue of it operating in a community of
cooperating peers, rather than a precisely defined authoritarian
hierarchy controlled by a strict chain of formal delegations from the
root. These differences are summarized below:
Multicast DNS...
* uses multicast
* uses UDP port 5353 instead of port 53
* operates in well-defined parts of the DNS namespace
* uses UTF-8, and only UTF-8, to encode resource record names
* defines a clear limit on the maximum legal domain name
(256 bytes including final terminating root label zero byte)
* allows name compression in rdata for SRV and other record types
* allows larger UDP packets
* allows more than one question in a query packet
* uses the Answer Section of a query to list Known Answers
* uses the TC bit in a query to indicate additional Known Answers
* uses the Authority Section of a query for probe tie-breaking
* ignores the Query ID field (except for generating legacy responses)
* doesn't require the question to be repeated in the response packet
* uses gratuitous responses to announce new records to the peer group
* uses NSEC records to signal non-existence of records
* defines a "unicast response" bit in the rrclass of query questions
* defines a "cache flush" bit in the rrclass of response answers
* uses DNS TTL 0 to indicate that a record has been deleted
* recommends AAAA records in the additional section when responding
to rrtype "A" queries, and vice versa
* monitors queries to perform Duplicate Question Suppression
* monitors responses to perform Duplicate Answer Suppression...
* ... and Ongoing Conflict Detection
* ... and Opportunistic Caching
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23. Benefits of Multicast Responses
Some people have argued that sending responses via multicast is
inefficient on the network. In fact using multicast responses can
result in a net lowering of overall multicast traffic for a variety
of reasons, and provides other benefits too:
* One multicast response can update the cache on all machines on the
network. If another machine later wants to issue the same query, it
already has the answer in its cache, so it may not need to even
transmit that multicast query on the network at all.
* When more than one machine has the same ongoing long-lived query
running, every machine does not have to transmit its own
independent query. When one machine transmits a query, all the
other hosts see the answers, so they can suppress their own
queries.
* When a host sees a multicast query, but does not see the
corresponding multicast response, it can use this information
to promptly delete stale data from its cache. To achieve the
same level of user-interface quality and responsiveness without
multicast responses would require lower cache lifetimes and more
frequent network polling, resulting in a higher packet rate.
* Multicast responses allow passive conflict detection. Without this
ability, some other conflict detection mechanism would be needed,
imposing its own additional burden on the network.
* When using delayed responses to reduce network collisions, clients
need to maintain a list recording to whom each answer should be
sent. The option of multicast responses allows clients with limited
storage, which cannot store an arbitrarily long list of response
addresses, to choose to fail-over to a single multicast response in
place of multiple unicast responses, when appropriate.
* In the case of overlayed subnets, multicast responses allow a
receiver to know with certainty that a response originated on the
local link, even when its source address may apparently suggest
otherwise.
* Link-local multicast transcends virtually every conceivable network
misconfiguration. Even if you have a collection of devices where
every device's IP address, subnet mask, default gateway, and DNS
server address are all wrong, packets sent by any of those devices
addressed to a link-local multicast destination address will still
be delivered to all peers on the local link. This can be extremely
helpful when diagnosing and rectifying network problems, since
it facilitates a direct communication channel between client and
server that works without reliance on ARP, IP routing tables, etc.
Being able to discover what IP address a device has (or thinks it
has) is frequently a very valuable first step in diagnosing why it
is unable to communicate on the local network.
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24. IPv6 Considerations
An IPv4-only host and an IPv6-only host behave as "ships that pass in
the night". Even if they are on the same Ethernet, neither is aware
of the other's traffic. For this reason, each physical link may have
*two* unrelated ".local." zones, one for IPv4 and one for IPv6.
Since for practical purposes, a group of IPv4-only hosts and a group
of IPv6-only hosts on the same Ethernet act as if they were on two
entirely separate Ethernet segments, it is unsurprising that their
use of the ".local." zone should occur exactly as it would if
they really were on two entirely separate Ethernet segments.
A dual-stack (v4/v6) host can participate in both ".local."
zones, and should register its name(s) and perform its lookups both
using IPv4 and IPv6. This enables it to reach, and be reached by,
both IPv4-only and IPv6-only hosts. In effect this acts like a
multi-homed host, with one connection to the logical "IPv4 Ethernet
segment", and a connection to the logical "IPv6 Ethernet segment".
24.1 IPv6 Multicast Addresses by Hashing
Some discovery protocols use a range of multicast addresses, and
determine the address to be used by a hash function of the name being
sought. Queries are sent via multicast to the address as indicated by
the hash function, and responses are returned to the querier via
unicast. Particularly in IPv6, where multicast addresses are
extremely plentiful, this approach is frequently advocated.
There are some problems with this:
* When a host has a large number of records with different names, the
host may have to join a large number of multicast groups. This can
place undue burden on the Ethernet hardware, which typically
supports a limited number of multicast addresses efficiently. When
this number is exceeded, the Ethernet hardware may have to resort
to receiving all multicasts and passing them up to the host
software for filtering, thereby defeating the point of using a
multicast address range in the first place.
* Multiple questions cannot be placed in one packet if they don't all
hash to the same multicast address.
* Duplicate Question Suppression doesn't work if queriers are not
seeing each other's queries.
* Duplicate Answer Suppression doesn't work if Responders are not
seeing each other's responses.
* Opportunistic Caching doesn't work.
* Ongoing Conflict Detection doesn't work.
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25. Security Considerations
The algorithm for detecting and resolving name conflicts is, by its
very nature, an algorithm that assumes cooperating participants. Its
purpose is to allow a group of hosts to arrive at a mutually disjoint
set of host names and other DNS resource record names, in the absence
of any central authority to coordinate this or mediate disputes. In
the absence of any higher authority to resolve disputes, the only
alternative is that the participants must work together cooperatively
to arrive at a resolution.
In an environment where the participants are mutually antagonistic
and unwilling to cooperate, other mechanisms are appropriate, like
manually administered DNS.
In an environment where there is a group of cooperating participants,
but there may be other antagonistic participants on the same physical
link, the cooperating participants need to use IPSEC signatures
and/or DNSSEC [RFC 2535] signatures so that they can distinguish mDNS
messages from trusted participants (which they process as usual) from
mDNS messages from untrusted participants (which they silently
discard).
When DNS queries for *global* DNS names are sent to the mDNS
multicast address (during network outages which disrupt communication
with the greater Internet) it is *especially* important to use
DNSSEC, because the user may have the impression that he or she is
communicating with some authentic host, when in fact he or she is
really communicating with some local host that is merely masquerading
as that name. This is less critical for names ending with ".local.",
because the user should be aware that those names have only local
significance and no global authority is implied.
Most computer users neglect to type the trailing dot at the end of a
fully qualified domain name, making it a relative domain name (e.g.
"www.example.com"). In the event of network outage, attempts to
positively resolve the name as entered will fail, resulting in
application of the search list, including ".local.", if present.
A malicious host could masquerade as "www.example.com." by answering
the resulting Multicast DNS query for "www.example.com.local."
To avoid this, a host MUST NOT append the search suffix
".local.", if present, to any relative (partially qualified)
host name containing two or more labels. Appending ".local." to
single-label relative host names is acceptable, since the user
should have no expectation that a single-label host name will
resolve as-is.
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26. IANA Considerations
IANA has allocated the IPv4 link-local multicast address 224.0.0.251
for the use described in this document.
IANA has allocated the IPv6 multicast address set FF0X::FB
for the use described in this document. Only address FF02::FB
(Link-Local Scope) is currently in use by deployed software,
but it is possible that in future implementers may experiment
with Multicast DNS using larger-scoped addresses, such as FF05::FB
(Site-Local Scope) [RFC 4291].
When this document is published, IANA should designate a list of
domains which are deemed to have only link-local significance, as
described in Section 12 of this document ("Special Characteristics of
Multicast DNS Domains").
The re-use of the top bit of the rrclass field in the Question and
Answer Sections means that Multicast DNS can only carry DNS records
with classes in the range 0-32767. Classes in the range 32768 to
65535 are incompatible with Multicast DNS. However, since to-date
only three DNS classes have been assigned by IANA (1, 3 and 4),
and only one (1, "Internet") is actually in widespread use, this
limitation is likely to remain a purely theoretical one.
No other IANA services are required by this document.
27. Acknowledgments
The concepts described in this document have been explored, developed
and implemented with help from Freek Dijkstra, Erik Guttman, Paul
Vixie, Bill Woodcock, and others.
Special thanks go to Bob Bradley, Josh Graessley, Scott Herscher,
Rory McGuire, Roger Pantos and Kiren Sekar for their significant
contributions.
28. Deployment History
Multicast DNS client software first became available to the public
in Mac OS 9 in 2001. Multicast DNS Responder software first began
shipping to end users in large volumes with the launch of Mac OS X
10.2 Jaguar in August 2002, and became available for Microsoft
Windows users with the launch of Apple's "Rendezvous for Windows"
(now "Bonjour for Windows") in June 2004 [B4W].
Apple released the source code for the mDNSResponder daemon as Open
Source in September 2002, first under Apple's standard Apple Public
Source License, and then later, in August 2006, under the Apache
License, Version 2.0.
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In addition to desktop and laptop computers running Mac OS X and
Microsoft Windows, Multicast DNS is implemented in a wide range of
hardware devices, such as Apple's "AirPort Extreme" and "AirPort
Express" wireless base stations, home gateways from other vendors,
network printers, network cameras, TiVo DVRs, etc.
The Open Source community has produced many independent
implementations of Multicast DNS, some in C like Apple's
mDNSResponder daemon, and others in a variety of different languages
including Java, Python, Perl, and C#/Mono.
29. Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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30. Normative References
[RFC 1034] Mockapetris, P., "Domain Names - Concepts and
Facilities", STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation and
Specifications", STD 13, RFC 1035, November 1987.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[RFC 3845] Schlyter, J., "DNS Security (DNSSEC) NextSECure (NSEC)
RDATA Format", RFC 3845, August 2004.
[UAX15] "Unicode Normalization Forms"
31. Informative References
[B4W] Bonjour for Windows
[BPF] Berkeley Packet Filter
[djbdl]
[DNS-SD] Cheshire, S., and M. Krochmal, "DNS-Based Service
Discovery", Internet-Draft (work in progress),
draft-cheshire-dnsext-dns-sd-05.txt, September 2008.
[IEEE 802] IEEE Standards for Local and Metropolitan Area Networks:
Overview and Architecture.
Institute of Electrical and Electronic Engineers,
IEEE Standard 802, 1990.
[IEEE W]
[ATalk] Cheshire, S., and M. Krochmal,
"Requirements for Replacing AppleTalk",
Internet-Draft (work in progress),
draft-cheshire-dnsext-nbp-06.txt, September 2008.
[OWNER] Cheshire, S., et al., "EDNS0 OWNER option",
Internet-Draft (work in progress),
draft-cheshire-edns0-owner-option-00.txt, July 2009.
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[RFC 2052] Gulbrandsen, A., et al., "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2782, October 1996.
[RFC 2132] Alexander, S., and Droms, R., "DHCP Options and BOOTP
Vendor Extensions", RFC 2132, March 1997.
[RFC 2136] Vixie, P., et al., "Dynamic Updates in the Domain Name
System (DNS UPDATE)", RFC 2136, April 1997.
[RFC 2181] Elz, R., and Bush, R., "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC 2462] S. Thomson and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC 2606] Eastlake, D., and A. Panitz, "Reserved Top Level DNS
Names", RFC 2606, June 1999.
[RFC 2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC 2845] Vixie, P., et al., "Secret Key Transaction Authentication
for DNS (TSIG)", RFC 2845, May 2000.
[RFC 2860] Carpenter, B., Baker, F. and M. Roberts, "Memorandum
of Understanding Concerning the Technical Work of the
Internet Assigned Numbers Authority", RFC 2860, June
2000.
[RFC 2930] Eastlake, D., "Secret Key Establishment for DNS
(TKEY RR)", RFC 2930, September 2000.
[RFC 2931] Eastlake, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[RFC 3492] Costello, A., "Punycode: A Bootstring encoding of
Unicode for use with Internationalized Domain Names
in Applications (IDNA)", RFC 3492, March 2003.
[RFC 3927] Cheshire, S., B. Aboba, and E. Guttman,
"Dynamic Configuration of IPv4 Link-Local Addresses",
RFC 3927, May 2005.
[RFC 4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[WoL] Wake-on-LAN Magic Packet
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32. Authors' Addresses
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: cheshire@apple.com
Marc Krochmal
Apple Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 4368
EMail: marc@apple.com
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